Fxhash to rustchash

[KGrewal1]

This removes the internal code for fxhash and instead uses the fxhash crate and subsequently migrates use of the fxhash crate (https://crates.io/crates/fxhash) to the rustc-hash crate (https://crates.io/crates/rustc-hash) implementing the same hash function: I believe the latter will need to be audited however: also manually implement the hash convenience function from fxhash --- @fitzgen