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_mir_build/src/build/matches/test.rs

@@ -397,7 +397,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             (Some((region, elem_ty, _)), _) | (None, Some((region, elem_ty, _))) => {
                 let tcx = self.tcx;
                 // make both a slice
-                ty = tcx.mk_imm_ref(region, tcx.mk_slice(*elem_ty));
+                ty = tcx.mk_imm_ref(*region, tcx.mk_slice(*elem_ty));
                 if opt_ref_ty.is_some() {
                     let temp = self.temp(ty, source_info.span);
                     self.cfg.push_assign(