Guide macros and unsafe

The last two sections of the guide, and a small conclusion. I suck at conclusions.

I also realized I never covered strings, so I'm going to put that section up before we're actually 'done.'
I find this paragraph a little confusing, by which I mean that the interface of Rust's `sync::Mutex` is actually entirely safe, it just uses unsafe under the hood. So maybe it'd be better to rephrase this to talk about how the `unsafe` keyword allows you to expose safe compositions of operations that would be unsafe in isolation.