feat: add feature to use rustc-hash (#423)

[obi1kenobi]

We use rustc-hash::FxHashMap/Set on src/visibility_tracker.rs this is
an implementation detail as AFAIK all types there are pub(crate) so we
never return an FxHashMap or FxHashSet on a public interface.

This bring a nice perf increase:

❯ cargo criterion --bench indexed_crate
IndexedCrate/new(aws-sdk-ec2)
                        time:   [1.3474 s 1.3506 s 1.3537 s]
                        change: [+4.4411% +4.8278% +5.2381%] (p = 0.00 < 0.05)
                        Performance has regressed.

❯ cargo criterion --bench indexed_crate --features rustc-hash
IndexedCrate/new(aws-sdk-ec2)
                        time:   [1.1555 s 1.1561 s 1.1567 s]
                        change: [-14.603% -14.405% -14.192%] (p = 0.00 < 0.05)
                        Performance has improved.

❯ cargo criterion --bench indexed_crate --features rayon
IndexedCrate/new(aws-sdk-ec2)
                        time:   [577.26 ms 578.45 ms 579.70 ms]
                        change: [-50.083% -49.964% -49.852%] (p = 0.00 < 0.05)
                        Performance has improved.

❯ cargo criterion --bench indexed_crate --features rayon,rustc-hash
IndexedCrate/new(aws-sdk-ec2)
                        time:   [486.94 ms 488.49 ms 490.13 ms]
                        change: [-15.862% -15.552% -15.212%] (p = 0.00 < 0.05)
                        Performance has improved.

Co-authored-by: Predrag Gruevski 2348618+obi1kenobi@users.noreply.github.com