Change confusing wording: null nodes compare as equal to null handles, the previous wording implied that each null handle is unique

This may or may not make it more clear that pugixml.cpp has to be
compiled - either as one of the projects or as a standalone project via
CMake et al - for it to work by default.

Fixes #256.

This is implicitly true due to the following section, but that was
written before C++11 so this does deserve a special mention in ranged
for section as well.

Fixes #210.

Mention ubsan fixes; these fixes probably fix compact mode on some
64-bit architecture where unaligned pointer reads aren't valid as well
but it's probably not very relevant...

Still trying to decide if the next version should be 1.9 or 1.8x and
what other changes need to go in.

<mesh> node attribute name is "name", not "mesh".

Fixes #176.

It has always been the case that pugixml does not perform Unicode
validation or name/tag Unicode character class validation, but it wasn't
very obvious from documentation.

Fixes #162

We support Latin-1 and automatically detect it by parsing the encoding
from document declaration; both of these were omitted from the
description of the automatic detection.

Additionally, the description has been rewritten to be more concise and
a bit more abstract - there's no need to specify the algorithm precisely
here.

Fixes #158.

Also add it to the changelog for 1.8

Git warns when it finds "whitespace errors". This commit gets
rid of these whitespace errors for code and adoc files.

Also remove the description of behavior for trailing non-numeric characters.
It's likely this will become a parse error in the future so better leave it
as unspecified for now.

Fixes #80.

Fixes #61.

Also add PUGIXML_COMPACT to pugiconfig.hpp