Better support for 32-bit systems

[glebm]

I'd like to use this with a project that targets a lot of legacy 32-bit hardware (https://github.com/diasurgical/devilutionX).

At the moment, unordered_dense explicitly uses 64-bit integers in a lot of places, such as the return type of the hash function, the to64 function, etc.

The standard library hash containers, on the other hand, use size_t for this purpose, which is 32-bit on 32-bit systems.

It'd be great if this library were more 32-bit friendly as well.