Add MurmurHash64A and MurmurHash64B to hv_func.h

Both of these hash functions are by Austin Appleby and are in the public domain.

The 64A variant is designed for 64 bit machines.

The 64B variant is designed for 32 bit machines.

Both use unaligned loads, so are unsuitable for platforms with strict alignment requirements.

Both have been converted to use Perls hash function calling conventions,
and to return a 32 bit hash instead of a 64 bit hash (low 32 bits)