Overhaul TyS and Ty.

Specifically, change `Ty` from this:
```
pub type Ty<'tcx> = &'tcx TyS<'tcx>;
```
to this
```
pub struct Ty<'tcx>(Interned<'tcx, TyS<'tcx>>);
```
There are two benefits to this.
- It's now a first class type, so we can define methods on it. This
  means we can move a lot of methods away from `TyS`, leaving `TyS` as a
  barely-used type, which is appropriate given that it's not meant to
  be used directly.
- The uniqueness requirement is now explicit, via the `Interned` type.
  E.g. the pointer-based `Eq` and `Hash` comes from `Interned`, rather
  than via `TyS`, which wasn't obvious at all.

Much of this commit is boring churn. The interesting changes are in
these files:
- compiler/rustc_middle/src/arena.rs
- compiler/rustc_middle/src/mir/visit.rs
- compiler/rustc_middle/src/ty/context.rs
- compiler/rustc_middle/src/ty/mod.rs

Specifically:
- Most mentions of `TyS` are removed. It's very much a dumb struct now;
  `Ty` has all the smarts.
- `TyS` now has `crate` visibility instead of `pub`.
- `TyS::make_for_test` is removed in favour of the static `BOOL_TY`,
  which just works better with the new structure.
- The `Eq`/`Ord`/`Hash` impls are removed from `TyS`. `Interned`s impls
  of `Eq`/`Hash` now suffice. `Ord` is now partly on `Interned`
  (pointer-based, for the `Equal` case) and partly on `TyS`
  (contents-based, for the other cases).
- There are many tedious sigil adjustments, i.e. adding or removing `*`
  or `&`. They seem to be unavoidable.

compiler/rustc_codegen_cranelift/src/value_and_place.rs

@@ -514,7 +514,7 @@ impl<'tcx> CPlace<'tcx> {
                     // Can only happen for vector types
                     let len =
                         u16::try_from(len.eval_usize(fx.tcx, ParamEnv::reveal_all())).unwrap();
-                    let vector_ty = fx.clif_type(element).unwrap().by(len).unwrap();
+                    let vector_ty = fx.clif_type(*element).unwrap().by(len).unwrap();
 
                     let data = match from.0 {
                         CValueInner::ByRef(ptr, None) => {
@@ -721,8 +721,8 @@ impl<'tcx> CPlace<'tcx> {
         index: Value,
     ) -> CPlace<'tcx> {
         let (elem_layout, ptr) = match self.layout().ty.kind() {
-            ty::Array(elem_ty, _) => (fx.layout_of(elem_ty), self.to_ptr()),
-            ty::Slice(elem_ty) => (fx.layout_of(elem_ty), self.to_ptr_maybe_unsized().0),
+            ty::Array(elem_ty, _) => (fx.layout_of(*elem_ty), self.to_ptr()),
+            ty::Slice(elem_ty) => (fx.layout_of(*elem_ty), self.to_ptr_maybe_unsized().0),
             _ => bug!("place_index({:?})", self.layout().ty),
         };
 
@@ -781,11 +781,11 @@ pub(crate) fn assert_assignable<'tcx>(
             ty::RawPtr(TypeAndMut { ty: a, mutbl: _ }),
             ty::RawPtr(TypeAndMut { ty: b, mutbl: _ }),
         ) => {
-            assert_assignable(fx, a, b);
+            assert_assignable(fx, *a, *b);
         }
         (ty::Ref(_, a, _), ty::RawPtr(TypeAndMut { ty: b, mutbl: _ }))
         | (ty::RawPtr(TypeAndMut { ty: a, mutbl: _ }), ty::Ref(_, b, _)) => {
-            assert_assignable(fx, a, b);
+            assert_assignable(fx, *a, *b);
         }
         (ty::FnPtr(_), ty::FnPtr(_)) => {
             let from_sig = fx.tcx.normalize_erasing_late_bound_regions(