Guide: iterators

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
`num` is a reference here, so maybe `println!("{}", *num);`. It's hard to tell in this case, since `Show` is implemented for `&int`, but it does trip people up quite a bit ("why does `.iter()` give me references?" isn't an uncommon question on IRC), so it's probably best to not gloss over it entirely.