Implement iterator for hash tables and sets

[Rot127]

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• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
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Detailed description

Adds iterators over (im)mutable values or immutable keys of hash tables and sets.

Supersedes #4500

Test plan

Added

Closing issues

...

[XVilka]

LGTM with a few nitpicks

[wargio]

LGTM, those comments are not required.