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/auto_trait.rs

@@ -809,14 +809,14 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                     };
                 }
                 ty::PredicateKind::ConstEquate(c1, c2) => {
-                    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() {
                             match select.infcx().const_eval_resolve(
                                 obligation.param_env,
                                 unevaluated,
                                 Some(obligation.cause.span),
                             ) {
-                                Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty)),
+                                Ok(val) => Ok(ty::Const::from_value(select.tcx(), val, c.ty())),
                                 Err(err) => Err(err),
                             }
                         } else {