This prevents Catch from disabling `Wpadded` for Clang inside test files
(files that do not define either `CATCH_CONFIG_MAIN` or
`CATCH_CONFIG_RUNNER`).

catch_suppress_warnings.h and catch_reenable_warnings.h should be
included only once*, so that the stitching script includes them as the
first and last header respectively, since it only includes each header
once. This caused a bug, where the first one was included properly, but
the second one was included prematurely, from catch_xmlwriter.hpp, and
thus was guarded by `CATCH_IMPL`.

* At least until the stitching script is changed to accomodate common
warning disabling header.

Fixes #871

This fixes an issue where XML reports on stdout are broken by printf statements

Provide an extension point on XmlReporter to be able to supply a stylesheet url in a derived implementation

Character encodings that are not valid in XML 1.0 are instead written using C-style escapes

This reverts commit a189387f49f129a3554c47426a0dfe176c5c0781.

Character encodings that are not valid in XML 1.0 are instead written using C-style escapes

In reality, this is a relatively small performance improvement,
especially with the previous improvements removing lots of superfluous
string handling, but still was measurable.

* Empty strings are now direct constructed as `std::string()`, not as empty string literals.
* `startsWith` and `endsWith` no longer construct new a string. This should be an improvement
for libstdc++ when using older standards, as it doesn't use SSO but COW and thus even short
strings are expensive to first create.
* Various places now use char literal instead of string literals containing single char.
** `startsWith` and `endsWith` now also have overload that takes single char.

Generally the performance improvements under VS2015 are small, as going from short string
to char is mostly meaningless because of SSO (Catch doesn't push string handling that hard)
and previous commit removed most string handling if tests pass, which is the expect case.