Add rayon support

[notgull]

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

[orlp]

Yes, I should add this under an optional feature.

[Calandiel]

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.

[DenialAdams]

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

[DenialAdams]

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