Removed nested `StdString` namespace and added Doxygen comments.
Also renamed some matchers to avoid colisions now that there are
less separate namespaces for matchers to go to. Since this is a
breaking release anyway, it shouldn't matter, and the factory
functions that the users should use remain the same anyway.
Outside of `MatcherBase` and `GenericMatcherBase`, matchers are not
designed to be overriden. This means that doing so can easily lead
to errors, and matchers are generally fairly simple functionality-wise.
so there is not much code reuse to be gained anyway.
Thus, Catch2-provided concrete matchers are now final.
