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_trait_selection/src/traits/select/mod.rs

@@ -643,7 +643,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         //
                         // Let's just see where this breaks :shrug:
                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
-                            (c1.val, c2.val)
+                            (c1.val(), c2.val())
                         {
                             if self.infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
                                 return Ok(EvaluatedToOk);
@@ -651,15 +651,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         }
                     }
 
-                    let evaluate = |c: &'tcx ty::Const<'tcx>| {
-                        if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
+                    let evaluate = |c: ty::Const<'tcx>| {
+                        if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
                             self.infcx
                                 .const_eval_resolve(
                                     obligation.param_env,
                                     unevaluated,
                                     Some(obligation.cause.span),
                                 )
-                                .map(|val| ty::Const::from_value(self.tcx(), val, c.ty))
+                                .map(|val| ty::Const::from_value(self.tcx(), val, c.ty()))
                         } else {
                             Ok(c)
                         }