Fix HashSet retain fn mut signature and make HashSet Iter pub

[4r33x]

HashSet Iter struct is needed for NewType patterns

[ibraheemdev]

Ah, I meant to do this as part of #80. I think I would prefer making the map and set modules public as papaya::map and papaya::set, and exporting any specific types under their respective modules, i.e., map::{Iter, Keys, Values, OccupiedError, Compute, HashMapRef, HashMapBuilder} and set::{HashSetRef, HashSetBuilder, Iter}, to be consistent with the standard library. We could remove the duplicate imports from root (as a breaking change).

[4r33x]

Rebased, made the map and set modules public, removed some map imports from the lib.rs, and addressed all Clippy lints (stable 1.94) in the tests except clippy::needless_range_loop

If you want, I can split clippy changes into separate PR

[4r33x]

@ibraheemdev Could you take a look, please?

[ibraheemdev]

Sorry I've been on vacation for a little while, I'll try to get to this soon.

[ibraheemdev]

Sorry for the wait. I split out the HashSet::retain change into #101 because the module reorganization is a breaking change. Can we also remove HashMapBuilder,HashSetBuilder, HashMapRef, and HashSetRef from the root exports, now that they are exported from the submodules?

[4r33x]

Removed, also bumped version to comply with CI