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_lint/src/builtin.rs

@@ -2896,25 +2896,21 @@ impl ClashingExternDeclarations {
                         (Array(a_ty, a_const), Array(b_ty, b_const)) => {
                             // For arrays, we also check the constness of the type.
                             a_const.val == b_const.val
-                                && structurally_same_type_impl(seen_types, cx, a_ty, b_ty, ckind)
+                                && structurally_same_type_impl(seen_types, cx, *a_ty, *b_ty, ckind)
                         }
                         (Slice(a_ty), Slice(b_ty)) => {
-                            structurally_same_type_impl(seen_types, cx, a_ty, b_ty, ckind)
+                            structurally_same_type_impl(seen_types, cx, *a_ty, *b_ty, ckind)
                         }
                         (RawPtr(a_tymut), RawPtr(b_tymut)) => {
                             a_tymut.mutbl == b_tymut.mutbl
                                 && structurally_same_type_impl(
-                                    seen_types,
-                                    cx,
-                                    &a_tymut.ty,
-                                    &b_tymut.ty,
-                                    ckind,
+                                    seen_types, cx, a_tymut.ty, b_tymut.ty, ckind,
                                 )
                         }
                         (Ref(_a_region, a_ty, a_mut), Ref(_b_region, b_ty, b_mut)) => {
                             // For structural sameness, we don't need the region to be same.
                             a_mut == b_mut
-                                && structurally_same_type_impl(seen_types, cx, a_ty, b_ty, ckind)
+                                && structurally_same_type_impl(seen_types, cx, *a_ty, *b_ty, ckind)
                         }
                         (FnDef(..), FnDef(..)) => {
                             let a_poly_sig = a.fn_sig(tcx);
@@ -2927,7 +2923,7 @@ impl ClashingExternDeclarations {
                             (a_sig.abi, a_sig.unsafety, a_sig.c_variadic)
                                 == (b_sig.abi, b_sig.unsafety, b_sig.c_variadic)
                                 && a_sig.inputs().iter().eq_by(b_sig.inputs().iter(), |a, b| {
-                                    structurally_same_type_impl(seen_types, cx, a, b, ckind)
+                                    structurally_same_type_impl(seen_types, cx, *a, *b, ckind)
                                 })
                                 && structurally_same_type_impl(
                                     seen_types,