Guide: iterators

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
Consuming an `Iterator` by calling its `next` repeatedly is still doing an iteration... At an introductory level, I find the distinction between the `Map` 'iterator' and the `map` 'iterator adapter' to just adding confusion, especially since you'll never really instantiate a `Map`.

That said, I like your two broad classes. Gonna work that in here somehow.