Tracking issue for 'Allow `Self` to appear in the where clause of trait impls'

RFC: https://github.com/rust-lang/rfcs/blob/master/text/1647-allow-self-in-where-clauses.md

RFC PR: https://github.com/rust-lang/rfcs/pull/1647

Proposes allowing the `Self` type to be used in every position in trait implementations, including where clauses and other parameters to the trait being implemented.