ty/walk: iterate `GenericArg`s instead of `Ty`s.
Before this PR, `Ty::walk` only iterated over `Ty`s, but that's becoming an increasing problem with `const` generics, as `ty::Const`s in `Substs` are missed by it.
By working with `GenericArg` instead, we can handle both `Ty`s and `ty::Const`s, but also `ty::Region`s, which used to require ad-hoc mechanisms such as `push_regions`.
I've also removed `TraitRef::input_types`, as it's both long obsolete, and easy to misuse.
209b2be09f