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_query_impl/src/keys.rs

@@ -275,7 +275,7 @@ impl<'tcx> Key for (ty::ParamEnv<'tcx>, ty::PolyTraitRef<'tcx>) {
     }
 }
 
-impl<'tcx> Key for (&'tcx ty::Const<'tcx>, mir::Field) {
+impl<'tcx> Key for (ty::Const<'tcx>, mir::Field) {
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {
         true
@@ -345,7 +345,7 @@ impl<'tcx> Key for mir::ConstantKind<'tcx> {
     }
 }
 
-impl<'tcx> Key for &'tcx ty::Const<'tcx> {
+impl<'tcx> Key for ty::Const<'tcx> {
     #[inline(always)]
     fn query_crate_is_local(&self) -> bool {
         true