PS invocation of code coverage upload gets triggered because of stderr
output from curl with progress report when uploading coverage.

In some MSVC versions on x64 configurations, the hashing function
triggers this failure:

Run-Time Check Failure #1 - A cast to a smaller data type has caused a
loss of data.  If this was intentional, you should mask the source of
the cast with the appropriate bitmask.

This is similar to the integer sanitizer - this code is valid C++ but
MSVC decides to warn about this nonetheless. Masking the pointer's low
32 bits fixes the issue.

Fixes #357.

Use nullptr in pugixml.hpp if available, see #91

pugixml currently unconditionally accepts documents with multiple
top-level element nodes in absence of parse_fragment. This is an
unfortunate omission; while it can be corrected, it will result in
regressions for some users, and it's trivial to perform the validity
check after the parse is done.

Because of this, for now we're just going to amend documentation here to
both highlight this in the W3C Conformance section, but also to more
strongly push users into realizing that there's just a single document
element (normally).

We might decide to change the behavior here to prohibit such documents
by default in the future, but for now a documentation change seems like
a better tradeoff.

Fixes #337

Various CMake improvements

This will allow projects to use pugixml from its build directory
rather than having to install it.

Add "Include pugixml.cpp" and "Header Only" linkage options

The old logic for BUILD_SHARED_LIBS and BUILD_SHARED_AND_STATIC_LIBS
would produce two targets with the same name and different build
rules. The Makefile generator tolerated this, but the Ninja generator
raised an error. Fix the logic so that only one shared and one static
target gets built, and make the pugixml target an ALIAS of the one
dictated by BUILD_SHARED_LIBS.

The loop traverses the source tree and simultaneously builds up a copy
of it at destination. Short of race conditions, this code is safe -
however, it's not obvious that dit stays inside the destination tree.

This change adds a few assertions to help enforce/document these
invariants. One particular subtlety is that dit can actually *become*
null after we exit out of the loop, but it's guaranteed to only do so
once sit goes back to sn.

This is only possible when doing a full document copy - for some reason
we weren't using this for that (in reset(xml_document)), but we are now.

Fixes #314.

We were previously relying on non-standard comment detection that is
supported by gcc/clang to avoid warnings about implicit fallthrough.

This can be solved using attributes but using them requires a lot of
compiler-specific detection logic because not all versions of gcc/clang
support them.

We don't *really* need to rely on fallthrough here - the type conversion
block can be located *after* the AST type switch instead, which means
that any AST type that has type ambiguity can fall back to that in the
future.

Fixes #331.

Update CMakeLists.txt to rename the static and shared libraries to pugixml

Don't call them pugixml-shared or pugixml-static, because this breaks pkgconfig calls and userspace tools.

This makes sure all conversions work as expected (note that no type can
be converted to node set so we don't check that).

Add POSITION_INDEPENDENT_CODE option

Allow to use the static library in other dynamic librairies. This is a standard cmake option https://cmake.org/cmake/help/v3.11/prop_tgt/POSITION_INDEPENDENT_CODE.html#prop_tgt:POSITION_INDEPENDENT_CODE

Instead of performing a late null check that is redundant and only
needed to silence clang static analysis warning, we pick the context as
a root / self node. This way the code is a bit less redundant and the
static analyzer is happy.

Fixing possible null pointer dereference

explicit casts for -Wconversions warnings in gcc

Fix incorrect installation location for exported CMake targets

Fix install locations for pkgconfig *and* cmake

Modernize CMake

The block of code can be removed in the future if CMake 3.15 ever
becomes the minimum

Move pugixml-config.cmake.in to scripts/ to prevent polluting the root

New Features:

pugixml-shared and pugixml-static are "always" available, but not always
built. This allows downstream projects to still use them, and to make
sure our configurations are correct regardless of if they are being
built. They are not always installed however.

pugixml-shared and pugixml-static have ALIAS libraries of
pugixml::shared and pugixml::static respectively. These names are also
respected in the `find_package(pugixml CONFIG)` generated files, so its
safe to use pugixml in a CMake project regardless of whether it is
installed locally, or if its a subproject via `add_subdirectory`.

pugixml will automatically select the correct library type based on
BUILD_SHARED_LIBS. A pugixml::pugixml ALIAS is also available.

CMAKE_MSVC_RUNTIME_LIBRARY from CMake 3.15 has been backported.

CMake will now rely on generator expressions (a powerful abstraction in
large projects with many subdirectories) for most work. This offloads
work from the single-threaded configure stage to the multithreaded
generation stage.

pugixml now uses CTest as the runner.

Some settings are automatically disabled if pugixml is used as a
subdirectory. These are still able to be manually set, but are hidden
from folks who choose to use CMake GUI.

 Bump CMake minimum to 3.4
 pugixml no longer requires a C compiler to be found or set to
compile correctly. This speeds up configuration and building on windows.
 Begin laying groundwork to backport MSVC_RUNTIME_LIBRARY
property

Fixes #285 

This keeps src/ folder clean of auxiliary files only required for
special builds; note that CMakeLists.txt already depends on scripts/
(specifically for pkgconfig setup).

Closes #294 

The newly added tests make sure that during node/attribute destruction
we deallocate a few memory pages; this makes sure that we don't read
node data after it's being destroyed.

Also clean up formatting/style in the remove_* implementation a bit.

These functions remove all attributes / child nodes in bulk which is faster than removing them one at a time.