Minimal version of TAITs

[nikomatsakis]

TAIT presently has distinct behavior in the old and new trait solver, primarily in edge cases. In the 2023 types team meetup, we identified a subset of TAIT that we believe could be stabilized promptly, sidestepping these edge cases.

[oli-obk]

I added all the remaining items to the TODO list of the TAIT stabilization github project: https://github.com/orgs/rust-lang/projects/22/views/1

[oli-obk]

We're focussing on stabilizing opaque types in associated types of trait impls. These have much clearer scoping rules (just the impl). See rust-lang/rust#119766