rust-performance: introduce FastHashMap type alias for HashMap Rust's default hashing is slow, because it is meant for preventing collision attacks. For all of the current Rust code, we don't care about those attacks, because if an person with bad intentions has write access to your repo, you have other issues. I've chosen to use the TwoXHash crate because it was made by a reputable member of the Rust community and has very good benchmarks. For now it does not seem to improve performance by much for the current code, but it's something else to not worry about when benchmarking code: in a previous experiment with copytracing in Rust, it accounted for more than 10% of the time of the entire script. Differential Revision: https://phab.mercurial-scm.org/D7116