Support `hashbrown` hash maps and sets with or without `use_std`

[aqrln]

Add an optional hashbrown feature that switches the underlying hash map implementation from std::collections to hashbrown and makes items that depend on hash maps available without use_std (although it still requires and implicitly enables use_alloc).

This addresses the motivating use case of #605 (which is the same use case that I am interested in) but does not implement a generic approach as the issue suggests. #901 would be a better solution long term and would make these changes obsolete, but the PR appears to be stalled and I wanted a simple solution that allows using APIs like unique/unique_by with no_std now.

It also partially addresses #322 because enabling this feature does switch to a faster hash algorithm, it just doesn't allow to specify which one exactly (currently the default hasher in hashbrown is foldhash, so this is what's going to be used until/unless changed upstream in the future).

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TODO:

• [ ] fix tests compilation with --all-features/-F hashbrown
• [ ] run tests on CI with both hash map implementations
• [ ] adjust contributor doc in the section about use_std

I'm opening a draft PR early because I'd like to get feedback on whether it's something you want to have.