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_borrowck/src/diagnostics/move_errors.rs

@@ -246,18 +246,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             );
             (
                 match kind {
-                    IllegalMoveOriginKind::BorrowedContent { target_place } => self
+                    &IllegalMoveOriginKind::BorrowedContent { target_place } => self
                         .report_cannot_move_from_borrowed_content(
                             original_path,
-                            *target_place,
+                            target_place,
                             span,
                             use_spans,
                         ),
-                    IllegalMoveOriginKind::InteriorOfTypeWithDestructor { container_ty: ty } => {
+                    &IllegalMoveOriginKind::InteriorOfTypeWithDestructor { container_ty: ty } => {
                         self.cannot_move_out_of_interior_of_drop(span, ty)
                     }
-                    IllegalMoveOriginKind::InteriorOfSliceOrArray { ty, is_index } => {
-                        self.cannot_move_out_of_interior_noncopy(span, ty, Some(*is_index))
+                    &IllegalMoveOriginKind::InteriorOfSliceOrArray { ty, is_index } => {
+                        self.cannot_move_out_of_interior_noncopy(span, ty, Some(is_index))
                     }
                 },
                 span,