Use `FxHashMap`?

This is much faster than the default hasher, and I don't think we're concerned about DOS attacks with this (if we are, could consider `AHashMap`)?

https://docs.rs/fxhash/latest/fxhash/type.FxHashMap.html

```rust
// use std::collections::HashMap;
use rustc_hash::{FxHashMap as HashMap};
```

If so, add linter:

https://nnethercote.github.io/perf-book/linting.html#disallowing-types