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.'
Discussing `println!` might be a good example? It does relatively complicated computation (compared to a C-like macro anyway), e.g. parsing & processing `"{foo:<5.8t} {0:03d}", x, foo = y`, and results in something that has a *clear* benefit: compile-time-checked flexible format strings.