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.
The [Oxford Comma](https://en.wikipedia.org/wiki/Oxford_comma) only applies to lists of three or more items. There are only two items here, so it doesn’t apply.

I think that the comma before ‘with’ should stay—it denotes a [non-restrictive](https://en.wikipedia.org/wiki/English_relative_clauses#Restrictive_or_non-restrictive_relative_clauses) relative clause in this context. (There is only one source file, so restricting what kind of source file it is is pointless. A non-restrictive clause only *adds* detail, which is what it is doing here.)

So, I think that this should be written as: ‘There are now two files: our source code, with the `.rs` extension, and the executable (`hello_world.exe` on Windows, `hello_world` everywhere else).’