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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/debuginfo/mod.rs b/src/debuginfo/mod.rs
index 8e203b8cfa0..693092ba543 100644
--- a/src/debuginfo/mod.rs
+++ b/src/debuginfo/mod.rs
@@ -114,7 +114,7 @@ impl<'tcx> DebugContext<'tcx> {
     }
 
     fn dwarf_ty(&mut self, ty: Ty<'tcx>) -> UnitEntryId {
-        if let Some(type_id) = self.types.get(ty) {
+        if let Some(type_id) = self.types.get(&ty) {
             return *type_id;
         }
 
@@ -143,7 +143,7 @@ impl<'tcx> DebugContext<'tcx> {
                 // Ensure that type is inserted before recursing to avoid duplicates
                 self.types.insert(ty, type_id);
 
-                let pointee = self.dwarf_ty(pointee_ty);
+                let pointee = self.dwarf_ty(*pointee_ty);
 
                 let type_entry = self.dwarf.unit.get_mut(type_id);