This is primarily done to support new `std::*_ordering` types,
but the refactoring also supports any other type with this
property.

The compilation overhead is surprisingly low. Testing it with
clang on a Linux machine, compiling our SelfTest project takes
only 2-3% longer with these changes than it takes otherwise.

Closes #2555

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

The previous implementation was always 1 number ahead. Fixing this
is not important at all, but it was bugging me.

Closes #2468

MinGW doesn't support `__try` and friends at all, while Clang
only supports it partially, and the test would require some
changes to make it work there. Since this is only a test, we can
afford to keep it MSVC-only.

Closes #2447

This is kinda messy, because there is no good way to signal to
the compiler that some code uses direct comparison of floating
point numbers intentionally, so instead we have to use diagnostic
pragmas.

We also have to over-suppress the test files, because Clang (and
possibly GCC) still issue warnings from template instantiation
even if the instantion site is under warning suppression, so the
template definition has to be under warning suppression as well.

Closes #2406