Random patches (incl. #[quickcheck] syntax extension)

I finally got around to bringing https://github.com/rust-lang/rust/pull/14100 upstream.

This includes @MrAlert's `#[quickcheck]` syntax extension (the first 2 commits) and a few other minor fixes.
Oh, and this `continue` isn't meant to be here... I didn't do a very good job, sorry @BurntSushi. :frowning: