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_mir_build/src/build/misc.rs

@@ -25,11 +25,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
     /// Convenience function for creating a literal operand, one
     /// without any user type annotation.
-    crate fn literal_operand(
-        &mut self,
-        span: Span,
-        literal: &'tcx ty::Const<'tcx>,
-    ) -> Operand<'tcx> {
+    crate fn literal_operand(&mut self, span: Span, literal: ty::Const<'tcx>) -> Operand<'tcx> {
         let literal = literal.into();
         let constant = Box::new(Constant { span, user_ty: None, literal });
         Operand::Constant(constant)