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.
Actually, even this still has a clarity issue that I want to avoid, which is that `x` refers to a *value* at a particular location, not to the location itself. So perhaps that should be

> When you create a new variable binding, you're giving a name to a value that's stored at a particular location on the stack. Like this:

This indicates that it's a value, but also that the value has a location, and that location is on the stack. This should allow you to subsequently talk about the location without confusion.