Split the repeated `SipHash` v[0..3] initialization into a small `SipHashState` helper that is used by both `CSipHasher` and `PresaltedSipHasher`.
Added explanatory comments to clarify behavior, documenting the equivalence of `PresaltedSipHasher` `operator()` overloads to `CSipHasher` usage.
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>