Auto merge of #40213 - steveklabnik:extract-reference, r=alexcrichton

Move the reference into a submodule

Part of https://github.com/rust-lang/rust/issues/39588

This moves the reference out into its own submodule. Right now, it's under my own account; I would assume that it should go under `rust-lang`, but I guess there's an argument for the nursery too? I can fix that up before we commit it.

r? @alexcrichton @brson