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'm just pointing out that writing `extern { fn foo(); }` (etc.) doesn't actually involve writing the `unsafe` *keyword* anywhere (not even as a "macro expansion" or anything), i.e. every `extern` function is automatically unsafe to call, but it hasn't actually got "`unsafe`" anywhere near it.

(That is, I'm not using "property" as a technical word, feel free to choose another or rephrase entirely.)