feat: add non-cryptographic hash function

[tychoish]

Part of #2220

Notes:

• While Rust's hashmaps use a siphash variant, they use a less less
  common and less robust variant (for speed) reasons [which makes
  sense for default hashmaps but less sense in other cases]. They also
  deprecated the access to the specific algorithims, in a way that
  makes me less confident that the undeprecated form will be
  semantically stable.

• I contemplated (and tried) to share the implementation, but: I think
  these ought to be different functions and becuause the hashers are
  stateful, I think we'd end up writing a lot more code with limited
  benefit, but perhaps I've missed something.