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.

clippy_lints/src/default_numeric_fallback.rs

@@ -123,7 +123,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                 if let Some(fn_sig) = fn_sig_opt(self.cx, func.hir_id) {
                     for (expr, bound) in iter::zip(*args, fn_sig.skip_binder().inputs()) {
                         // Push found arg type, then visit arg.
-                        self.ty_bounds.push(TyBound::Ty(bound));
+                        self.ty_bounds.push(TyBound::Ty(*bound));
                         self.visit_expr(expr);
                         self.ty_bounds.pop();
                     }
@@ -135,7 +135,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
                 if let Some(def_id) = self.cx.typeck_results().type_dependent_def_id(expr.hir_id) {
                     let fn_sig = self.cx.tcx.fn_sig(def_id).skip_binder();
                     for (expr, bound) in iter::zip(*args, fn_sig.inputs()) {
-                        self.ty_bounds.push(TyBound::Ty(bound));
+                        self.ty_bounds.push(TyBound::Ty(*bound));
                         self.visit_expr(expr);
                         self.ty_bounds.pop();
                     }
@@ -210,7 +210,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NumericFallbackVisitor<'a, 'tcx> {
 
 fn fn_sig_opt<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId) -> Option<PolyFnSig<'tcx>> {
     let node_ty = cx.typeck_results().node_type_opt(hir_id)?;
-    // We can't use `TyS::fn_sig` because it automatically performs substs, this may result in FNs.
+    // We can't use `Ty::fn_sig` because it automatically performs substs, this may result in FNs.
     match node_ty.kind() {
         ty::FnDef(def_id, _) => Some(cx.tcx.fn_sig(*def_id)),
         ty::FnPtr(fn_sig) => Some(*fn_sig),