The old internals reached into the global hub to stash the allocation(s)
for enum value -> string value there, then kept around a potentially
invalid (in case the hub was cleaned up) reference into it, going through
whole bunch of virtual dispatch in the process.

The new internals just store the data in a static variable inside the
`StringMaker` specialization. This avoids potential lifetime issues,
avoids all virtual dispatch and (almost) reduces the include bloat in
the main header path.

The reason for (almost) there is that for full include correctness,
`EnumInfo` needs `<utility>` include for `std::pair`. However, this brings
in things like `std::relops`, because the std headers in C++ are dumb.
As this was not included before, and instead we relied on `std::pair`
existing in an internal stdlib header that we transitively included, the
full include size ends up bigger than before.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

After everything else was split out, this name much reflects the
actual contents of the file(s).

Note that only documentation and comments are impacted by this change.

This should also be done for test files, but that has lower priority.

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.

Doing some benchmarking with ClangBuildAnalyzer suggests that
compiling Catch2's `SelfTest` spends 10% of the time instantiating
`std::unique_ptr` for some interface types required for registering
and running tests.

The lesser compilation overhead of `Catch::Detail::unique_ptr` should
significantly reduce that time.

The compiled implementation was also changed to use the custom impl,
to avoid having to convert between using `std::unique_ptr` and
`Catch::Detail::unique_ptr`. This will likely also improve the compile
times of the implementation, but that is less important than improving
compilation times of the user's TUs with tests.