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.'
Yeah, it's a good note. I think I'm going to just ignore that detail for now, so I don't have to get into it. Once you say 'rust doesn't have x,' you have to explain why...