Implement nth and nth_back for list and tuple iterators

[ngoldbaum]

Right now BoundListIterator::nth will call next n times (I think, please correct me if I'm misinterpreting), when instead it could get the nth item immediately. We should override Iterator::nth and DoubleEndedIterator::nth_back on stable rust and Iterator::advance_by and DoubleEndedIterator::advance_back_by on nightly rust to do constant-time indexing for all of the list and tuple iterator stucts.

[bschoenmaeckers]

Same can be done with last()