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/mir/visit.rs

@@ -194,7 +194,7 @@ macro_rules! make_mir_visitor {
             }
 
             fn visit_region(&mut self,
-                            region: & $($mutability)? ty::Region<'tcx>,
+                            region: $(& $mutability)? ty::Region<'tcx>,
                             _: Location) {
                 self.super_region(region);
             }
@@ -641,7 +641,7 @@ macro_rules! make_mir_visitor {
                     Rvalue::ThreadLocalRef(_) => {}
 
                     Rvalue::Ref(r, bk, path) => {
-                        self.visit_region(r, location);
+                        self.visit_region($(& $mutability)? *r, location);
                         let ctx = match bk {
                             BorrowKind::Shared => PlaceContext::NonMutatingUse(
                                 NonMutatingUseContext::SharedBorrow
@@ -900,7 +900,7 @@ macro_rules! make_mir_visitor {
             fn super_ty(&mut self, _ty: $(& $mutability)? Ty<'tcx>) {
             }
 
-            fn super_region(&mut self, _region: & $($mutability)? ty::Region<'tcx>) {
+            fn super_region(&mut self, _region: $(& $mutability)? ty::Region<'tcx>) {
             }
 
             fn super_const(&mut self, _const: & $($mutability)? &'tcx ty::Const<'tcx>) {