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 think this is supposed to showcase the power of the macro system but it doesn't seem that way to me. It just looks like regex is implemented with macros rather than functions. With functions you could something like this where the returned structure has a function called `is_match` to test with.

```rust
let date_matcher = regex(r"^\d{4}-\d{2}-\d{2}$");
println!("{}", date_matcher.is_match("2014-01-01"));
```

Is it easy to show why it is much better implemented as a macro than a function? If so, this seems like a good place for it.