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/conflict_errors.rs

@@ -2324,7 +2324,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 // This is also case 2 from above but for functions, return type is still an
                 // anonymous reference so we select the first argument.
                 let argument_span = fn_decl.inputs.first()?.span;
-                let argument_ty = sig.inputs().skip_binder().first()?;
+                let argument_ty = *sig.inputs().skip_binder().first()?;
 
                 let return_span = fn_decl.output.span();
                 let return_ty = sig.output().skip_binder();
@@ -2379,27 +2379,27 @@ impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
         diag: &mut DiagnosticBuilder<'_>,
     ) -> String {
         match self {
-            AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
+            &AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
                 diag.span_label(
-                    *argument_span,
+                    argument_span,
                     format!("has type `{}`", cx.get_name_for_ty(argument_ty, 0)),
                 );
 
                 cx.get_region_name_for_ty(argument_ty, 0)
             }
-            AnnotatedBorrowFnSignature::AnonymousFunction {
+            &AnnotatedBorrowFnSignature::AnonymousFunction {
                 argument_ty,
                 argument_span,
                 return_ty,
                 return_span,
             } => {
                 let argument_ty_name = cx.get_name_for_ty(argument_ty, 0);
-                diag.span_label(*argument_span, format!("has type `{}`", argument_ty_name));
+                diag.span_label(argument_span, format!("has type `{}`", argument_ty_name));
 
                 let return_ty_name = cx.get_name_for_ty(return_ty, 0);
                 let types_equal = return_ty_name == argument_ty_name;
                 diag.span_label(
-                    *return_span,
+                    return_span,
                     format!(
                         "{}has type `{}`",
                         if types_equal { "also " } else { "" },
@@ -2419,7 +2419,7 @@ impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
             }
             AnnotatedBorrowFnSignature::NamedFunction { arguments, return_ty, return_span } => {
                 // Region of return type and arguments checked to be the same earlier.
-                let region_name = cx.get_region_name_for_ty(return_ty, 0);
+                let region_name = cx.get_region_name_for_ty(*return_ty, 0);
                 for (_, argument_span) in arguments {
                     diag.span_label(*argument_span, format!("has lifetime `{}`", region_name));
                 }