Use a faster hash function

[kmcallister]

e.g. rustc's FnvHasher

[SimonSapin]

Depends on rust-phf/rust-phf#88

[SimonSapin]

Something to try rust-phf/rust-phf#392, though it changes the signatures of HashState and get_index, so our PhfStrSet and codegen would need to change accordingly.