Overhaul RegionKind and Region.

Specifically, change `Region` from this:
```
pub type Region<'tcx> = &'tcx RegionKind;
```
to this:
```
pub struct Region<'tcx>(&'tcx Interned<RegionKind>);
```

This now matches `Ty` and `Predicate` more closely.

Things to note
- Regions have always been interned, but we haven't been using pointer-based
  `Eq` and `Hash`. This is now happening.
- I chose to impl `Deref` for `Region` because it makes pattern matching a lot
  nicer, and `Region` can be viewed as just a smart wrapper for `RegionKind`.
- Various methods are moved from `RegionKind` to `Region`.
- There is a lot of tedious sigil changes.
- A couple of types like `HighlightBuilder`, `RegionHighlightMode` now have a
  `'tcx` lifetime because they hold a `Ty<'tcx>`, so they can call `mk_region`.
- A couple of test outputs change slightly, I'm not sure why, but the new
  outputs are a little better.

compiler/rustc_ty_utils/src/instance.rs

@@ -90,8 +90,8 @@ impl<'tcx> TypeVisitor<'tcx> for BoundVarsCollector<'tcx> {
     }
 
     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
-        match r {
-            ty::ReLateBound(index, br) if *index == self.binder_index => {
+        match *r {
+            ty::ReLateBound(index, br) if index == self.binder_index => {
                 match self.vars.entry(br.var.as_u32()) {
                     Entry::Vacant(entry) => {
                         entry.insert(ty::BoundVariableKind::Region(br.kind));

compiler/rustc_ty_utils/src/ty.rs

@@ -410,7 +410,7 @@ fn issue33140_self_ty(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Ty<'_>> {
 
     let self_ty = trait_ref.self_ty();
     let self_ty_matches = match self_ty.kind() {
-        ty::Dynamic(ref data, ty::ReStatic) => data.principal().is_none(),
+        ty::Dynamic(ref data, re) if re.is_static() => data.principal().is_none(),
         _ => false,
     };