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.
s/all/some/ (There is/will be more Rust than just `rustc`/`cargo`, e.g. we currently have `rustdoc`, and I'm sure people will be writing others.)