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.
I'm loath to mention `GC` here (or at all, in this guide). Use of the current "GC" shouldn't be encouraged IMO, and there's not a guarantee we will have a sane GC by 1.0 (or ever).

I would prefer it if you could just talk about generalised `box` as a future improvement, possibly with a single mention of `box(GC) x` just as an example, but with caveats.