feat: verify keys method on HashMaps

[monsterkrampe]

This PR verifies the keys function on Std.HashMap.

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Initial discussions have already happend with @TwoFX and we are collaborating on this matter.
This will remain a draft as long as not all desired results have been added.

If we should still create an issue for the topic of this PR, let us know.
Of course, any other feedback is appreciated as well :)

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 62521f4f2d12ad239644e160dfdc5bd4d34d3f1b --onto 9847923f9be5de968f10ed7c7493e3ca0072abce. (2024-10-28 15:44:59)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 62521f4f2d12ad239644e160dfdc5bd4d34d3f1b --onto c779f3a039963fd38b03a78f635f0a7c36f24f42. (2024-11-05 17:43:47)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 345ecd20c9d8c435fd1ac05d4a239af170f76f7a --onto c779f3a039963fd38b03a78f635f0a7c36f24f42. (2024-11-06 16:23:08)

[monsterkrampe]

I think this is ready for review now.
Let us know if other results around keys would be expected :)

Also, I'm currently not sure why one of the tests fails. Maybe this would be resolved by a rebase...

[TwoFX]

Great work, thanks!