Uplift `TypeVisitableExt` into `rustc_type_ir`
This uplifts `TypeVisitableExt` into `rustc_type_ir` so it can be used in an interner-agnostic way. It also moves some `TypeSuperVisitable` bounds onto `Interner` since we don't expect to support libraries that have types which aren't foldable by default.
This restores a couple of asserts in the canonicalizer code we uplifted, and also makes it so that we can use type-flags-based helpers in the solver code, which I'm interested in uplifting.
r? lcnr
cc54612ac3