[stdlib][WIP] Recursive where clause constraints

[airspeedswift]

Add protocol where clauses that require things like protocol Collection where SubSequence: Collection

Builds on top of #8990, no new test failures.

[airspeedswift]

@DougGregor it seems if I write them as where clauses on protocols, I can add recursive constraints on things like Collection.SubSequence: Collection that compile and work fine. I think this is all we need to remove the need for std lib users to have to specify unnecessary constraints.

[airspeedswift]

turns out we don't even need the prior PRs – this can be done directly into master. Re-raised as #9374