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_ty_utils/src/representability.rs

@@ -92,7 +92,7 @@ fn are_inner_types_recursive<'tcx>(
             seen,
             shadow_seen,
             representable_cache,
-            ty,
+            *ty,
             force_result,
         ),
         ty::Adt(def, substs) => {
@@ -255,7 +255,7 @@ fn is_type_structurally_recursive<'tcx>(
     force_result: &mut bool,
 ) -> Representability {
     debug!("is_type_structurally_recursive: {:?} {:?}", ty, sp);
-    if let Some(representability) = representable_cache.get(ty) {
+    if let Some(representability) = representable_cache.get(&ty) {
         debug!(
             "is_type_structurally_recursive: {:?} {:?} - (cached) {:?}",
             ty, sp, representability