compile time enum reflection

[Tsche]

The technique used by magic_enum and librepr::ctei should be explained. Since librepr::ctei outperforms magic_enum quite significantly, the optimization techniques used should also be explained.

[Tsche]

I implemented an experimental builtin for this in Clang a while back, not sure whether this is in scope of a post like that. It would definitely require lots of cleaning up.