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.'
No, I meant in your list of items:
> 1. It parses the string to find any {}s
2. It checks that the number of {}s matches the numer of other arguments.
3. It generates a bunch of Rust code, taking this in mind.

4.. Look, it allows overloading! We couldn't do that before; the number of `{}` isn't set!

Your call. Just pointing it out.