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_privacy/src/lib.rs

@@ -280,8 +280,8 @@ where
         }
     }
 
-    fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-        self.visit_ty(c.ty)?;
+    fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+        self.visit_ty(c.ty())?;
         let tcx = self.def_id_visitor.tcx();
         if let Ok(Some(ct)) = AbstractConst::from_const(tcx, c) {
             self.visit_abstract_const_expr(tcx, ct)?;