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.

clippy_lints/src/methods/expect_fun_call.rs

@@ -73,7 +73,7 @@ pub(super) fn check<'tcx>(
                     match cx.qpath_res(p, fun.hir_id) {
                         hir::def::Res::Def(hir::def::DefKind::Fn | hir::def::DefKind::AssocFn, def_id) => matches!(
                             cx.tcx.fn_sig(def_id).output().skip_binder().kind(),
-                            ty::Ref(ty::ReStatic, ..)
+                            ty::Ref(re, ..) if re.is_static(),
                         ),
                         _ => false,
                     }
@@ -87,7 +87,7 @@ pub(super) fn check<'tcx>(
                     .map_or(false, |method_id| {
                         matches!(
                             cx.tcx.fn_sig(method_id).output().skip_binder().kind(),
-                            ty::Ref(ty::ReStatic, ..)
+                            ty::Ref(re, ..) if re.is_static()
                         )
                     })
             },