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.'
http://doc.rust-lang.org/master/guide-unsafe.html is relevant.

I think this section should emphasise that code inside `unsafe` doesn't change existing language semantics and still needs to follow the rules/[avoid bad things](http://doc.rust-lang.org/master/rust.html#behavior-considered-unsafe); the code is just offered a little more power (e.g. by being able to manipulate `*` pointers).