Hello, is there a reason [as_unsigned](https://github.com/intel/cpp-std-extensions/blob/ff6101764730d2131e403288d5bb8cf629c147af/include/stdx/utility.hpp#L100) and [as_signed](https://github.com/intel/cpp-std-extensions/blob/ff6101764730d2131e403288d5bb8cf629c147af/include/stdx/utility.hpp#L107) cannot be `constexpr`?
