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_lint/src/builtin.rs

@@ -2895,7 +2895,7 @@ impl ClashingExternDeclarations {
                         }
                         (Array(a_ty, a_const), Array(b_ty, b_const)) => {
                             // For arrays, we also check the constness of the type.
-                            a_const.val == b_const.val
+                            a_const.val() == b_const.val()
                                 && structurally_same_type_impl(seen_types, cx, *a_ty, *b_ty, ckind)
                         }
                         (Slice(a_ty), Slice(b_ty)) => {

compiler/rustc_lint/src/context.rs

@@ -974,7 +974,7 @@ impl<'tcx> LateContext<'tcx> {
                 Ok(())
             }
 
-            fn print_const(self, _ct: &'tcx ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
+            fn print_const(self, _ct: ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
                 Ok(())
             }