- 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.)
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