extend explainer to cover iterator/lending-iterator split

[nikomatsakis]

@jackh726 raised the point on Zulip that we ought to work out how we will approach "GAT-ifying" existing traits, with iterators being a very prominent example.

After some discussion, we felt the best way to start was to try to extend the explainer with some code examples of how we would like for people to be able to use iterators -- then we can try to figure out how to make that possible.