Guide Redux: Pointers

This is super, super WIP, but I'm going to go get lunch for a while, and figured I'd toss my work up here in case anyone wants to see my work as I do it.

This contains a new introductory section explaining the basics of pointers, and some pitfalls that Rust attempts to solve. I'd be interested in hearing how my explanation is, as well as if this belongs here. Pointers are such a crucial concept, I don't mind having a beginners' section on them in the main docs, even though our main audience is supposed to understand them already. Reasonable people may disagree, however.
The generalised design for `box` hasn't been worked out, and, it's a bit useless to give an example of generalised box when the only other thing it can create is not recommended for use.

I'm happy for it to be a single example, but it's not a very good type at the moment (e.g. auto-magic reference counts whenever you pass it around by value, no nice move semantics to elide them like with `Rc`) and (I think) gets too much air-time in our docs (both old and new).