8402: Remove Ty::substs{_mut} r=flodiebold a=flodiebold
Almost all uses actually only care about ADT substs, so it's better to be explicit. The methods were a bad abstraction anyway since they already didn't include the inner types of e.g. `TyKind::Ref` anymore.
Co-authored-by: Florian Diebold <flodiebold@gmail.com>