Overhaul Const.

Specifically, rename the `Const` struct as `ConstS` and re-introduce `Const` as
this:
```
pub struct Const<'tcx>(&'tcx Interned<ConstS>);
```
This now matches `Ty` and `Predicate` more closely, including using
pointer-based `eq` and `hash`.

Notable changes:
- `mk_const` now takes a `ConstS`.
- `Const` was copy, despite being 48 bytes. Now `ConstS` is not, so need a
  we need separate arena for it, because we can't use the `Dropless` one any
  more.
- Many `&'tcx Const<'tcx>`/`&Const<'tcx>` to `Const<'tcx>` changes
- Many `ct.ty` to `ct.ty()` and `ct.val` to `ct.val()` changes.
- Lots of tedious sigil fiddling.

compiler/rustc_const_eval/src/interpret/util.rs

@@ -55,7 +55,7 @@ where
                                     assert!(matches!(ty.kind(), ty::Param(_)))
                                 }
                                 ty::subst::GenericArgKind::Const(ct) => {
-                                    assert!(matches!(ct.val, ty::ConstKind::Param(_)))
+                                    assert!(matches!(ct.val(), ty::ConstKind::Param(_)))
                                 }
                                 ty::subst::GenericArgKind::Lifetime(..) => (),
                             },
@@ -68,8 +68,8 @@ where
             }
         }
 
-        fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-            match c.val {
+        fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+            match c.val() {
                 ty::ConstKind::Param(..) => ControlFlow::Break(FoundParam),
                 _ => c.super_visit_with(self),
             }