Implement FusedIterator for our iterators.

[ChrisJr404]

Closes #16.

This implements FusedIterator for all the collection iterators. FusedIterator has been stable for a long time now, so the "when it stabilizes" caveat no longer applies.

Once each of the base iterators (IterPtr, the queue's LazilyReversedListIter, and the red-black tree's RangeIterPtr) returns None it keeps returning None, so the marker is sound. The public Iter, IterKeys, IterValues and RangeIter types are core::iter::Map wrappers over those base iterators, so they pick up FusedIterator automatically once the base ones have it.

I added a small "keeps returning None" test per collection, plus assert_impl_all! checks on the public iterator types so the trait stays wired up.