Remove hashbrown dependency.

[mstange]

The std HashMap uses hashbrown these days, so I think this won't make anything slower. But I haven't tested it.

[calixteman]

There is a small difference between the hashing algorithm:

• https://doc.rust-lang.org/std/collections/struct.HashMap.html#
• https://docs.rs/hashbrown/latest/hashbrown/struct.HashMap.html#

And if I remember correctly, I chose the second because it was slightly faster and because we don't care about HashDoS attacks.
Anyway, it's just for you information, and you're free to do whatever you want :).

[mstange]

Ah, thanks for checking!

Then maybe we can use the regular HashMap with a different hasher, or even FxHashMap.

I'll do a speed comparison when I get a chance. For now we can just ignore this PR.
My main interest in removing the hashbrown dependency is so that we can stop getting dependabot PRs for it which would leave us with two or three copies of hashbrown.