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.
No, the slash would be interpreted very differently in cmd. I think, offhand, it'd treat it as ".<wordbreak>/hello_world", which is pretty nonsensical since . is a directory.

In the DOS/Windows world, / is the usual character for the start of a command argument, and it's special cased as such so that you can do like ```dir/s``` to get a recursive directory listing.