Apply consteval on appropriate basic_hashed_string related functions

[DNKpp]

It's easy to introduce unnecessary runtime-overhead by accident. The documentation even states this:

No string-to-number conversion will take place at runtime if used carefully.

Applying consteval on the two members, which accept an c-array, seems appropriate as they are obviously intended for compile-time construction.
Making the literals entt::literal::_hs and _hws consteval too, because there is literally no reason to use them at runtime.