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/project.rs

@@ -697,7 +697,7 @@ impl<'tcx> TypeFolder<'tcx> for PlaceholderReplacer<'_, 'tcx> {
     }
 
     fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        let r1 = match r0 {
+        let r1 = match *r0 {
             ty::ReVar(_) => self
                 .infcx
                 .inner