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_codegen_cranelift/src/base.rs

@@ -79,7 +79,7 @@ pub(crate) fn codegen_fn<'tcx>(
     let arg_uninhabited = fx
         .mir
         .args_iter()
-        .any(|arg| fx.layout_of(fx.monomorphize(&fx.mir.local_decls[arg].ty)).abi.is_uninhabited());
+        .any(|arg| fx.layout_of(fx.monomorphize(fx.mir.local_decls[arg].ty)).abi.is_uninhabited());
 
     if !crate::constant::check_constants(&mut fx) {
         fx.bcx.append_block_params_for_function_params(fx.block_map[START_BLOCK]);
@@ -818,16 +818,16 @@ pub(crate) fn codegen_place<'tcx>(
                 match cplace.layout().ty.kind() {
                     ty::Array(elem_ty, _len) => {
                         assert!(!from_end, "array subslices are never `from_end`");
-                        let elem_layout = fx.layout_of(elem_ty);
+                        let elem_layout = fx.layout_of(*elem_ty);
                         let ptr = cplace.to_ptr();
                         cplace = CPlace::for_ptr(
                             ptr.offset_i64(fx, elem_layout.size.bytes() as i64 * (from as i64)),
-                            fx.layout_of(fx.tcx.mk_array(elem_ty, to - from)),
+                            fx.layout_of(fx.tcx.mk_array(*elem_ty, to - from)),
                         );
                     }
                     ty::Slice(elem_ty) => {
                         assert!(from_end, "slice subslices should be `from_end`");
-                        let elem_layout = fx.layout_of(elem_ty);
+                        let elem_layout = fx.layout_of(*elem_ty);
                         let (ptr, len) = cplace.to_ptr_maybe_unsized();
                         let len = len.unwrap();
                         cplace = CPlace::for_ptr_with_extra(