fix: Optimize nth implementation for legacy UnorderedMap

[austinabell]

Made the change minimally invasive since this is somewhat of a legacy collection. The new ones are optimized and I assumed that zip with the optimization done to #634 would be applied to the UnorderedMap, but it wasn't.

exposes unordered_map module to allow Iter type to be used. Didn't add iterator types to others since no demand. There would be a benefit (in that the aux iterator traits would be available) but I don't want to return vector::Iter for backwards compatibility if we wanted to switch later and didn't want to just add a bunch of redundant code for no strong reason.

[ChaoticTempest]

interesting, so without this optimization, skip would've loaded in the elements twice over due to calling into nth twice and nth always loaded all the elements up to n? That's a pretty big win if someone were still using the old collections 👏

[austinabell]

interesting, so without this optimization, skip would've loaded in the elements twice over due to calling into nth twice and nth always loaded all the elements up to n? That's a pretty big win if someone were still using the old collections 👏

Yeah, zip for some reason avoids calling nth on the inner iterator and just instead calls next n times