Add pub(restricted) example

[cramertj]

See rust-lang/rust#32409
cc @steveklabnik

[cramertj]

(Travis is obviously going to fail since the feature isn't stable yet.)

[steveklabnik]

Yup! RBE deploys to master always, so we'll have to wait until the release to merge.

[cramertj]

To add:

• path in pub(in path) item must be some parent module of the item.
• pub(super) and pub(self) are shortcuts for pub(in super) and pub(in self) respectively
• pub(self) is the same thing as private

Edit: is it even worth mentioning pub(self)? why is that useful?

[steveklabnik]

We'll be able to merge this Thursday; it now is green on beta 🎊

[steveklabnik]

Woo!