Guide iterators

This isn't ready to merge yet.

The 'containers and iterators' guide is basically just a collection of stuff that should be in the module definitions. So I'm moving the guide to just an 'iterators' guide, and moved the info that was there into the right places.

So, is this a good path forward, and is all of the information still correct?
Ahh. I was looking in the wrong place, because that paragraph comes form here: https://github.com/rust-lang/rust/pull/16887/files#diff-cc3af746394f582ee863741589439e75L45

I don't remember specifically why, may even just be my own error.