perf: use rustc-hash for HashMap and HashSet

[Cheong-Lau]

Since we already depend on rustc-hash transiently, this doesn't add any more dependencies. As long as DOS attacks aren't a concern (which I don't think they are?), this should be free performance.

In my (admittedly naive) testing, this really improved CPU usage in some cases, which is pretty nice to get for free.

Also, on a separate note, I would like to work on improving the performance more, but right now I don't actually know how to benchmark properly (cargo-flamegraph is returning nonsense for me?), so I've just been making random changes until performance improved. This is the first change I made that actually did anything meaningful 🫠

[mmstick]

Are you able to use samply? Did you set debug = true on your release profile? Note that Debian packaging automatically strips debug symbols.

https://github.com/mstange/samply

[Cheong-Lau]

Are you able to use samply?

I will check that out, thanks!

Did you set debug = true on your release profile?

Yeah, I was using the release-with-debug profile. cargo-flamegraph had a bunch of libc functions and didn't really have anything relevant to the Rust code I could see, so I'm not sure what's happening there. 😕
EDIT: maybe I wasn't giving perf the permissions it needed? hmm

[Cheong-Lau]

@mmstick I got it working! Just needed to give perf the perms

Anyways, a quick benchmark opening a large folder (~2000 items)

Current main	rustc-hash (this PR)	rustc-hash + other random optimisations
~197 ms	~192 ms	~180 ms
current_main.json.gz	rustc_hash.json.gz	secret_sauce.json.gz

rustc-hash isn't really helping here, which is fair enough. It made the app feel snappier elsewhere, so I might benchmark something else later. But it means my other optimisations weren't for nothing! I'll keep working on them and open a new PR later.