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’d prefer this written “our source code with the `.rs` extension, and the executable (`hello_world.exe` on Windows, `hello_world` everywhere else).” Then also, as noted earlier, we can remove the comment in the following code block.

(Now let’s debate Oxford comma!)