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_middle/src/query/mod.rs

@@ -1146,33 +1146,33 @@ rustc_queries! {
         desc { "computing whether `{}` is `Copy`", env.value }
         remap_env_constness
     }
-    /// Query backing `TyS::is_sized`.
+    /// Query backing `Ty::is_sized`.
     query is_sized_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Sized`", env.value }
         remap_env_constness
     }
-    /// Query backing `TyS::is_freeze`.
+    /// Query backing `Ty::is_freeze`.
     query is_freeze_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is freeze", env.value }
         remap_env_constness
     }
-    /// Query backing `TyS::is_unpin`.
+    /// Query backing `Ty::is_unpin`.
     query is_unpin_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` is `Unpin`", env.value }
         remap_env_constness
     }
-    /// Query backing `TyS::needs_drop`.
+    /// Query backing `Ty::needs_drop`.
     query needs_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` needs drop", env.value }
         remap_env_constness
     }
-    /// Query backing `TyS::has_significant_drop_raw`.
+    /// Query backing `Ty::has_significant_drop_raw`.
     query has_significant_drop_raw(env: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
         desc { "computing whether `{}` has a significant drop", env.value }
         remap_env_constness
     }
 
-    /// Query backing `TyS::is_structural_eq_shallow`.
+    /// Query backing `Ty::is_structural_eq_shallow`.
     ///
     /// This is only correct for ADTs. Call `is_structural_eq_shallow` to handle all types
     /// correctly.