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.

- it was forward declared as a class, which caused warnings on some compilers. It should really have been a class anyway.
- this addresses the same issue as PR #534, albeit from the other angle.

(syntactic sugar for AllOf, AnyOf and Not compositional matchers, respectively)

(replaces need for PRs #310 and #504)