Improve support for key sizes other than 32-bit.

[tpn]

We recently added a stop-gap measure to support 64-bit key sizes as long as they could be downsized. However, this results in a _pext_u64() bitmap being applied to the incoming key and casting the result into a 32-bit ULONG, then hashing that key as normal.

It would be useful to have native hash routines for each 8, 16, 32 and 64 bit key sizes for the cases where key downsizing can't be achieved.