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.'
There isn't function overloading is there? You could never do this but it works with macros right? Seems like a feature.
```rust
println("{}", x);
println("{}, {}", x, y);
```