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.'
This isn't really the case. It's more things like "destructors should run exactly once" and "you need special types to mutate behind a `&`" and "`&mut` references shouldn't alias". It is true that Rust's way to enforce this is via memory naturally being uniquely owned, but it's not the fundamental reason (just a consequence).

I'm not sure if there's a pithy way to rephrase this though.