Guide: iterators

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
As I said below, I'd prefer the method to be called an iterator adaptor; in my mind, an 'iterator' is really the object that implements `Iterator`, e.g. `vec.iter()` returns an iterator, but the method `map` isn't one itself.

(If you were talking about [the type `Map`](http://doc.rust-lang.org/master/std/iter/struct.Map.html), or specifically "The first function that return an iterator we'll talk about is `map`"... but that's a bit awkward.)