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/query/normalize.rs

@@ -140,8 +140,8 @@ impl<'tcx> TypeVisitor<'tcx> for MaxEscapingBoundVarVisitor {
         ControlFlow::CONTINUE
     }
 
-    fn visit_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-        match ct.val {
+    fn visit_const(&mut self, ct: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        match ct.val() {
             ty::ConstKind::Bound(debruijn, _) if debruijn >= self.outer_index => {
                 self.escaping =
                     self.escaping.max(debruijn.as_usize() - self.outer_index.as_usize());
@@ -324,8 +324,8 @@ impl<'cx, 'tcx> FallibleTypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
 
     fn try_fold_const(
         &mut self,
-        constant: &'tcx ty::Const<'tcx>,
-    ) -> Result<&'tcx ty::Const<'tcx>, Self::Error> {
+        constant: ty::Const<'tcx>,
+    ) -> Result<ty::Const<'tcx>, Self::Error> {
         let constant = constant.try_super_fold_with(self)?;
         Ok(constant.eval(self.infcx.tcx, self.param_env))
     }