Previously we would check if the tag is special to validate that it isn't
reserved, and then to actually get the test case properties. We also did
the first check in particularly terrible order, leading to a lot of wasted
work.

The new implementation does property extraction while checking validity,
and is structured to avoid as much work as possible by early checks.

By avoiding `std::set`, we save 1 allocation per checked test case,
and the sort + pairwise check has much nicer cache behaviour, leading
to much faster check.

Overall, the check is only a tiny part of the startup overhead, but
the improvement is large enough that it ends up measurable in binaries
with many tests (e.g. `SelfTest`).

Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.

It is unlikely that this would ever come in practice, but there
is no reason to fix it.

Related to #2798

Mainly just removes some unused includes, but sometimes the include
is replaced by a smaller header instead.

Also removes a duplicated test case checking for empty tag error.

Related to #2650

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1