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/transform/promote_consts.rs

@@ -839,7 +839,7 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                     span,
                     user_ty: None,
                     literal: tcx
-                        .mk_const(ty::Const {
+                        .mk_const(ty::ConstS {
                             ty,
                             val: ty::ConstKind::Unevaluated(ty::Unevaluated {
                                 def,