Docs for associated refinements

[ranjitjhala]

Per @enjhnsn2 's suggestion -- I agree we'd want to have some docs on associated refinements; what is a good progression?

Is there something simpler than the iterator/vec-slice-index/ that folks have in mind?

[nilehmann]

I think implementing Index for a custom container is a good starting point (e.g. RVec) so you don't need to deal with the various indirections of the Vec implementation.

[nilehmann]

Hmm, but that requires modifying an external definition (adding an assoc refinement to Index). It would be good to have an example where everything is local