fix: iterate on elements

[SkArchon]

Description

This PR is based off of the PR here #460, I have opened a new PR as this only contains a subset of the changes, and I do not have access to push to the original authors fork.

This PR only contains the iteration function, as well as instead of introducing a new type for the Key and changing the API, we use any.

cc: @ilyatotl, @mangalaman93

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ SkArchon
❌ Ilia Merochkin

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[SkArchon]

@mangalaman93 Please do let me know if this PR is sufficient for merged based on your earlier comment here #460 (review).

I'm assuming based on the CLAssistant, @ilyatotl will need to sign the CLA, if not I might have to squash his commits.

[matthewmcneely]

@SkArchon While I can appreciate the usefulness of a first class iterator, I'm worried about the increase in overall memory usage this PR will introduce. 16 bytes for the OriginalKey field, which represents a 20% increase in memory use. Not trivial. My guess is this is why the original authors left it out.

Could you not roll-your-own by embedding a Cache inside your own struct? Implement Set and Get and store the keys in a Map, then provide the Iter func from there?