where the where: GAT syntax

[nikomatsakis]

Summary

Proposed: to alter the syntax of where clauses on type aliases so that they appear after the value:

type StringMap<K> = BTreeMap<K, String>
where
    K: PartialOrd

This applies both in top-level modules and in trats (associated types, generic or otherwise).

Background reading

https://rust-lang.github.io/generic-associated-types-initiative/design-discussions/where-the-where.html

rust-lang/rust#89122

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds
to a possible topic of discussion that may be scheduled for deeper discussion
during one of our design meetings.

[nikomatsakis]

cc @jackh726

[nikomatsakis]

Scheduled for Oct 13

[nikomatsakis]

Meeting notes: https://github.com/rust-lang/lang-team/blob/master/design-meeting-minutes/2021-10-13-where-the-where.md