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_middle/src/ty/util.rs

@@ -5,8 +5,9 @@ use crate::ty::fold::{FallibleTypeFolder, TypeFolder};
 use crate::ty::layout::IntegerExt;
 use crate::ty::query::TyCtxtAt;
 use crate::ty::subst::{GenericArgKind, Subst, SubstsRef};
-use crate::ty::TyKind::*;
-use crate::ty::{self, DebruijnIndex, DefIdTree, List, Ty, TyCtxt, TypeFoldable};
+use crate::ty::{
+    self, DebruijnIndex, DefIdTree, List, ReEarlyBound, Region, Ty, TyCtxt, TyKind::*, TypeFoldable,
+};
 use rustc_apfloat::Float as _;
 use rustc_ast as ast;
 use rustc_attr::{self as attr, SignedInt, UnsignedInt};
@@ -390,7 +391,7 @@ impl<'tcx> TyCtxt<'tcx> {
         let result = iter::zip(item_substs, impl_substs)
             .filter(|&(_, k)| {
                 match k.unpack() {
-                    GenericArgKind::Lifetime(&ty::RegionKind::ReEarlyBound(ref ebr)) => {
+                    GenericArgKind::Lifetime(Region(Interned(ReEarlyBound(ref ebr), _))) => {
                         !impl_generics.region_param(ebr, self).pure_wrt_drop
                     }
                     GenericArgKind::Type(Ty(Interned(