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.
As above, variables refer to values, not to addresses. So this should probably say

> [...] `x`, the name we're using for our variable, corresponds to the value at the memory location `0xd3e010`, and the value at that location is `5`.

My only discomfort here is the fact that the location actually depends entirely on the state of the stack when the function has entered, but I think it's ok to ignore that here. People who understand the stack will already understand that to be the case, and people who don't know how the stack works don't really need to care that the location can change.