An identifier containing a starting underscore followed by a capital
letter is reserved by the standard. It's trivial to avoid this by moving
the underscore to the end of the identifier.
While the likelihood of clashing here being minimal, we can turn a
"should not break" scenario into a definitive "will not break" one, so
why not?.
- For `std::same_as`, add missing include of `<concepts>`.
- For `std::convertible_to`, create a replacement in `common/concepts.h`
and use that instead.
This would also be found in `<concepts>`, but unlike `std::same_as`,
`std::convertible_to` is not yet implemented in libc++, LLVM's STL
implementation - not even in master. (In fact, `std::same_as` is the
*only* concept currently implemented. For some reason.)