refactor rustc-hash integration I found that rustc-hash is used in multiple compiler crates. Also some types use `FxBuildHasher` whereas others use `BuildHasherDefault<FxHasher>` (both do the same thing). In order to simplify future hashing experiments, I changed every location to use `rustc_data_structures::fx::*` types instead, and also removed the `BuildHasherDefault` variant. This will simplify future experiments with hashing (for example trying out a hasher that doesn't implement `Default` for whatever reason).