Most of these will not matter in practice due to C++14 imposing
significant limitations on what else we can make constexpr, and we cannot
have references outliving the constexpr context either way.
The previously used `make_unsigned` approach combined with the overload
set of `extendedMult` caused compilation issues on MacOS platform. By
forcing the selection to be one of `std::uintX_t` types we don't need
to complicate the overload set further.
Martin Hořeňovský