Implement the notion of a "generated unsafe block"

This way syntax extensions can generate unsafe blocks without worrying about them generating unnecessary unsafe warnings. Perhaps a special keyword could be added to be used in macros, but I don't think that's the best solution.

Currently if you use `format!` and friends in an `unsafe` block you're guaranteed to get some unused-unsafe warnings which is unfortunate. We normally do want these warnings, but I'm ok ignoring them in the case of compiler-generated unsafe blocks. I tried to do this in the least intrusive way possible, but others may have better ideas about how to do this.