Fix ICE that @steveklabnik encountered in rust-ice. The problems turned out to be...

Fix ICE that @steveklabnik encountered in rust-ice. The problems turned out to be that were being very loose with bound regions in trans (we were basically just ignoring and flattening binders). Since binders are significant to subtyping and hence to trait selection, this can cause a lot of problems. So this patch makes us treat them more strictly -- for example, we propagate binders, and avoid skipping past the `Binder` by writing `foo.0`.

Fixes #20644.