Use rustc-hash?

Hashing doesn't dominate our performance whatsoever, but might be a small boost to use a non-HashDoS-resistant hashmap. Doing so would be fine w/r/t security, since DoS isn't within our security model.

The issue names no files, tests, or entry points. Start by locating hashmap usage in the Rust encoder and decoder, then compare the current implementation with rustc-hash using relevant performance measurements. Done means establishing whether the change provides a worthwhile boost without exceeding the stated security model.

Written by the indexing model from the issue text.