Hello cargo

Built on top of https://github.com/rust-lang/rust/pull/15181. https://github.com/steveklabnik/rust/commit/e52719291183d7b4f691a1997c69fd75b8101b07 is the only new commit, you may want to review that separately.

I'm not sure if the car analogy is too glib.
I wonder whether it’s worthwhile putting in some form of “don’t be scared, our macros are safe” message here—to the person who has experience of other macro systems, using macros at this basic a level might scare them?

Not sure.