Guide: pointers

This is the next section of the guide, and it's on pointers. It's not done yet, as I need to write the section on ownership and borrowing, but I figured I'd share the rest now, to get feedback on the rest of it while I take some time to get that right.
If it's transferred, it's still unique: there's only ever one owner.

> It really helped me when learning Rust to have a lot of explicit type annotations. 

Yeah, this is an interesting point. I'm torn: I can see how being explicit with types can be helpful, but I _also_ want to encourage good Rust style, which would be to leave them off. You know?

Maybe this is a good place for comments? `// y is of type &mut Box<int>`?