Apart from being clearer, it also improves the overall codesize
of the implementation library, and should improve the performance
as well, by removing one level of indirection.
This is not nearly all of them, because IWYU does not support the
way Catch2 manages includes -- it expects that non-system includes
are done using `#include "foo/bar/baz.hpp"`, while Catch2 uses
`<foo/bar/baz.hpp>`. This causes trouble, because IWYU suggests
removing every single internal header, and then adding them again,
but using `""` in the include directive... the resulting suggestions
cannot be used without a lot of manual work, as they are largely
bogus.
For bonus points, IWYU also _loves_ to suggest kinda-random stdlib
headers for `size_t` and similar. Still, the resulting inclusion
graph is somewhat better than it was before.
