update for immovable types

[steveklabnik]

This issue tracks inclusion of the new pin types into the Book. This will happen in the 2018 edition.

Tracking issue: rust-lang/rust#49150

[RalfJung]

I'd prefer if we wouldn't call them "immovable types", that has already lead to confusion.

[steveklabnik]

Given that this is a stdlib change, I don't think we'll be including it directly in RBE.