Incorporate xxHash and ntHash as optional hashing algorithms

[edawson]

MurmurHash, while relatively fast / dispersive / backwards compatible with Mash, is slower than some newer algorithms. Moving to xxHash should yield a ~2X speed improvement in the hashing portions of the code, and ntHash should go even faster.

[luizirber]

Wearing my sourmash hat here:
I think we can bring this up with Mash and support ntHash, there is a field for specifying what hash is being used in the mash JSON schema.

But before going crazy on supporting every hash function under the sun it would be useful to have some agreement to keep everything compatible.

P.S: we also would need to agree on ntHash 1 or 2, see discussion here