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_middle/src/ty/fold.rs

@@ -960,10 +960,10 @@ impl<'tcx> TypeVisitor<'tcx> for ValidateBoundVars<'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 => {
                 if self.bound_vars.len() <= br.var.as_usize() {
-                    bug!("Not enough bound vars: {:?} not found in {:?}", *br, self.bound_vars);
+                    bug!("Not enough bound vars: {:?} not found in {:?}", br, self.bound_vars);
                 }
                 let list_var = self.bound_vars[br.var.as_usize()];
                 match list_var {
@@ -1076,9 +1076,9 @@ pub fn shift_region<'tcx>(
     region: ty::Region<'tcx>,
     amount: u32,
 ) -> ty::Region<'tcx> {
-    match region {
+    match *region {
         ty::ReLateBound(debruijn, br) if amount > 0 => {
-            tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), *br))
+            tcx.mk_region(ty::ReLateBound(debruijn.shifted_in(amount), br))
         }
         _ => region,
     }