Add instances for HashMap/HashSet

[janherich]

Description of the change
This PR adds At/Index instances for HashSet/HashMap from unordered-collections package.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation in the README and/or documentation directory
• Added a test for the contribution (if applicable)

[garyb]

I'm afraid this one is going to be declined, one of the rules for libraries in contrib is that they only depend on other libraries in contrib or on libraries in the core purescript organisation.

[janherich]

@garyb Got it, I'm very new to Purescript and that rule somehow evaded my attention.
Because orphaned typeclass instances are not allowed in Purescript, that severely limits usability of "generic" libraries (eq, generic datastructures, like purescript-unordered-collections) not in contrib packages.
I will try to discuss it on Discord, it seems like the correct way forward is to include purescript-unordered-collections in contrib, so I'm curious what are the blockers for that.

[garyb]

That's one option for sure, but ideally it would be nice to get the stuff sorted out so we have Hashable in the prelude too.