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_trait_selection/src/traits/auto_trait.rs

@@ -437,7 +437,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                     for (new_region, old_region) in
                         iter::zip(new_substs.regions(), old_substs.regions())
                     {
-                        match (new_region, old_region) {
+                        match (*new_region, *old_region) {
                             // If both predicates have an `ReLateBound` (a HRTB) in the
                             // same spot, we do nothing.
                             (
@@ -880,8 +880,8 @@ impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
     }
 
     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        (match r {
-            ty::ReVar(vid) => self.vid_to_region.get(vid).cloned(),
+        (match *r {
+            ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
             _ => None,
         })
         .unwrap_or_else(|| r.super_fold_with(self))