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.
Okay, I've pushed a language fix.

I've also made https://github.com/rust-lang/rust/issues/16143. I like this idea, but I want to see how it feels after the rest of the guide is complete.