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_mir_transform/src/required_consts.rs

@@ -15,7 +15,7 @@ impl<'a, 'tcx> RequiredConstsVisitor<'a, 'tcx> {
 impl<'tcx> Visitor<'tcx> for RequiredConstsVisitor<'_, 'tcx> {
     fn visit_constant(&mut self, constant: &Constant<'tcx>, _: Location) {
         if let Some(ct) = constant.literal.const_for_ty() {
-            if let ConstKind::Unevaluated(_) = ct.val {
+            if let ConstKind::Unevaluated(_) = ct.val() {
                 self.required_consts.push(*constant);
             }
         }