- 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.)