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.
In general, in windows command prompt (cmd.exe and I think powershell as well), the current directory is always in the search path so the ./ is unnecessary. And it'd be a backslash instead of a / if you did need to put path components.

Also, the .exe is part of the filename but it (along with .bat and some other less used extensions) are implicit and don't need to be put on.

So from that directory, in cmd, you should just have to type ```hello_world```.