Unsafe Fast Hash support

[kchalkias]

This will be required for benchmarking (similarly to empty/dummy signatures). It cannot empty (as collisions can break operations), maybe we can use the fastest non-cryptographically secure hash function for this purpose.

[jonas-lj]

Based on a bit of desktop research, the fastest such non cryptographic hash function seems to be xxHash3. This is based on the benchmarks they provide on their website and this discussion. If no-one objects, I'll give xxHash a go and do some basic benchmarks to verify.