Update the book to include info about implied bounds

[steveklabnik]

This issue tracks inclusion of information about implied bounds into the Book. This will happen in the 2018 edition.

Tracking issue: rust-lang/rust#44491

[steveklabnik]

This feature is not blocking the edition, retagging.

[steveklabnik]

We don't know when this will be stable, so let's decide when that happens.