Add rayon support

Rayon support for parallel iteration would be nice for when I want to iterate over a SlotMap in parallel.

Yes, I should add this under an optional feature.

Was there any work done in this direction since? I'm not sure how difficult it'd be but I could take a stab at it if it's still in the cards.

I am also looking for this functionality and would be happy to give it a try with some direction.

I created a pull request for this in #153. If anyone following this thread has the time, testing it out would be greatly appreciated :)

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