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_symbol_mangling/src/v0.rs

@@ -420,7 +420,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
                     hir::Mutability::Not => "R",
                     hir::Mutability::Mut => "Q",
                 });
-                if *r != ty::ReErased {
+                if !r.is_erased() {
                     self = r.print(self)?;
                 }
                 self = ty.print(self)?;
@@ -811,7 +811,7 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
     ) -> Result<Self::Path, Self::Error> {
         // Don't print any regions if they're all erased.
         let print_regions = args.iter().any(|arg| match arg.unpack() {
-            GenericArgKind::Lifetime(r) => *r != ty::ReErased,
+            GenericArgKind::Lifetime(r) => !r.is_erased(),
             _ => false,
         });
         let args = args.iter().cloned().filter(|arg| match arg.unpack() {