Given an unsorted sequence, remove_duplicates would sort it using the
pointer value of attributes/nodes and then remove consecutive
duplicates.

This was problematic because it meant that the result of XPath queries
was dependent on the memory allocation pattern. While it's technically
incorrect to rely on the order, this results in easy to miss bugs.

This is particularly common when XPath queries use union operators -
although we also will call remove_duplicates in other cases.

This change reworks the code to use a hash set instead, using the same
hash function we use for compact storage. To make sure it performs well,
we allocate enough buckets for count * 1.5 (assuming all elements are
unique); since each bucket is a single pointer unlike xpath_node which
is two pointers, we need somewhere between size * 0.75 and size * 1.5
temporary storage.

The resulting filtering is stable - we remove elements that we have seen
before but we don't change the order - and is actually significantly
faster than sorting was.

With a large union operation, before this change it took ~56 ms per 100
query invocations to remove duplicates, and after this change it takes
~20ms.

Fixes #254.

This does not change the result of a union operation [substantially], but
it means that we now give a list to remove_duplicates that has more natural
ordering.

If remove_duplicates didn't sort the array, we'd have union operations
resulting in a consistent predictable order.

Contributes to #254.

We had a few places in test code and library source where we used an
implicit float->double cast; while it should preserve the value exactly,
gcc/clang implement this warning to make sure uses of double are intentional.

This change also adds the warning to Makefile to make sure we don't
regress on this warning.

Fixes #243.

This change modifies the table entries for ctx_special_attr to treat TAB
character as special, which makes the output code escape it.

Before this change, trying to use TAB in an attribute value would output
it verbatim; during subsequent parsing, pugixml - and other compliant
parsers - would apply attribute-value normalization, turning the TAB
into a space and losing the original value.

Using 	 fixes this; if an input document has 	 in an attribute
value, that gets unescaped into \t during parsing and escaped back into
	 during output, which means we can now roundtrip values like this.

Fixes #242.

Coverity hits a similar false positive to what clang static analyzer hit
- it assumes that since optimize() checks _right for being nullptr,
optimize_self() might hit _right=nullptr in the ast_op_equal case which
is impossible.

Contributes to #236.

The following warning is removed:
Visual Studio 14.0
1. warning C4275: non dll-interface class 'std::exception' used as
   base for dll-interface class 'vtkpugixml::xpath_exception'

clang doesn't understand the invariants guaranteed for specific AST node
types and, when seeing null pointer checks in optimize(), assumes any
pointers in the node might be null. Work around this by adding explicit
- redundant - null pointer checks.

This change replaces xpath_node_set single element storage with a
single-element array in hopes that this would silence Coverity false
positive about getting a singleton pointer.

Additionally, it refactors _assign member to unify small and large
buffer codepaths since they are basically identical.

Fixes #233 (hopefully)

Fixes #224.

This might have helped a bit with confusion in #195.

gcc-8 produces "attribute directive ignored" warning for
no_sanitize("unsigned-integer-overflow"); at some point gcc will
introduce integer sanitizer support and we'll have to do this all over
again but for now just don't emit the attribute.

We were using << compact_alignment_log2 instead of * compact_alignment
for symmetry with the encoding where >> is crucial to keep code fast and
round to negative infinity.

For decoding, the results are the same and any reasonable compiler
should convert *4 into <<2 so just use a multiplication - that doesn't
trigger UB on negative numbers.

We were using allocate_memory to allocate struct xml_extra_buffer that
contains pointers; with compact mode, this allocation can be misaligned
by 4b with 8b pointers; fix this by manually realigning the pointer.

We were misaligning document data on 64-bit platforms by placing 8b
pointers at 4b offsets; fix this by reserving a full pointer worth of
bytes for page marker.

Define noexcept using _MSC_VER instead of _MSC_FULL_VER (first release
of MSVC 2015 should have it), remove redundant PUGIXML_HAS_NOEXCEPT and
define PUGIXML_NOEXCEPT_IF_NOT_COMPACT in terms of PUGIXML_NOEXCEPT.

* Adds noexcept specifiers to the move special members of xml_document, but only #ifndef PUGIXML_COMPACT

* Adds a macro definition to be able to use noexcept with supporting compilers

* Adds noexcept specifier to move special members of xpath_node_set, xpath_variable_set and xpath_query, but not of xml_document as it has a throwing implementation

Enables usage of override specifier for MSVC compilers (beginning with 17.0 which is the compiler of Visual Studio 2012)

Texas Instruments compiler produces this warning for unused template
member functions:

	"pugixml.cpp", line 253: warning #179-D: function
	"pugi::impl::<unnamed>::auto_deleter<T>::release [with
	T=pugi::impl::<unnamed>::xml_stream_chunk<char>]" was declared but
	never referenced

As far as I can tell, this is a compiler issue - these functions should
not be instantiated in the first place; while it's possible to rework
the code to work around this, the changes would be fragile. It seems
best to just disable this warning - we've seen something similar on SNC
(which appears to use the same frontend!..).

Fixes #182.

It looks like there are several cases where this might happen:

- In some MinGW distributions, the LLONG_MIN/etc defines are guarded
with:

	#if !defined(__STRICT_ANSI__) && defined(__GNUC__)

Which means that you don't get them in strict ANSI mode. The previous
workaround was specifically targeted towards this.

- In some GCC distributions (notably GCC 6.3.0 in some configurations),
LLONG_MIN/etc. defines are guarded with:

	#if (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)

But __STDC_VERSION__ isn't defined as C99 even if you use -std=c++14 -
which is probably technically valid, but not useful.

To work around this, redefine the symbols whenever we are building with
GCC and we need them and they aren't defined - doing this is better than
not building. Instead of hard-coding the constants, use GCC-specific
__LONG_LONG_MAX__ to compute them.

Fixes #181.

This makes it a bit faster and matches other internal code better.

The static_buffer optimization seems to come from the time where the
on-heap buffer was allocated using global memory operations. At this
point the temporary buffer and temporary string storage all come from
the evaluation stack (that can be partially allocated on heap...), so
the extra logic isn't relevant for performance.

In compact mode, we currently can not support zero-allocation moves
since some pointer assignments required during the move need to allocate
hash table slots.

This is mostly applicable to xml_document_struct::first_child, since the
pointer to this element is used as a hash table key, but there are some
contrived cases where parents of root's children need a hash slot and
didn't have it before.

These cases can be fixed by changing the compact encoding to be a bit
more move friendly, but for now it's easier to handle the error and
throw/return during move.

When this happens, the source document doesn't change.

This allows us to do a single reserve for a known amount of assignments
that is larger than the default minimum per reserve (16).

After move some nodes in the hash table can have keys that point to
other; this makes the table somewhat larger but this does not impact
correctness.

The reason is that for us to access a key in the hash table, there
should be a compact_pointer/string object with the state indicating that
it is stored in a hash table, and with the address matching the key. For
this to happen, we had to have put this object into this state which
would mean that we'd overwrite the hash entry with the new, correct
value.

When nodes/pages are being removed, we do not clean up keys from the
hash table - it's safe for the same reason, and thus move doesn't
introduce additional contracts here.

This change implements the initial version of move construction and
assignment support for documents.

When moving a document to another document, we always make sure move
target is in "clean" state (empty document), and proceed by relocating
all structures in the most efficient way possible.

Complications arise from the fact that the root (document) node is
embedded into xml_document object, so all pointers to it have to change;
this includes parent pointers of all first-level children as well as
allocator pointers in all memory pages and previous pointer in the first
on-heap memory page.

Additionally, compact mode makes everything even more complicated
because some of the pointers we need to update are stored in the hash
table (in fact, document first_child pointer is very likely to be there;
some parent pointers in first-level children will be using
compact_shared_parent but some won't be) which requires allocating a new
hash table which can fail.

Some details of this process are not fully fleshed out, especially for
compact mode; and this definitely requires many tests.

Clang/C2 does not implement __builtin_expect; additionally we need to
work around deprecation warnings for fopen by disabling them.

It's not clear whether we still need PUGI__MSVC_CRT_VERSION, but it's
more consistent for now to use it for _snprintf_s since this is relying
on a CRT extension, not on a compiler feature.

These functions were deprecated via comments in 1.5 but never got the
deprecated attribute; now is the time!

Using deprecated functions produces a warning; to silence it, this
change moves the relevant tests to a separate translation unit that has
deprecation disabled.

The macro only works correctly when the input argument is an array with
a statically known size - pointers or arrays decayed to pointers won't
work silently.

While this is unlikely to surface issues that aren't caught in
tests/code review, use _countof for MSVC to prevent such code from
compiling.

Rename partition to partition3 to resolve conflicts with std::partition.