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_middle/src/arena.rs

@@ -89,6 +89,7 @@ macro_rules! arena_types {
             // Interned types
             [] tys: rustc_middle::ty::TyS<'tcx>,
             [] predicates: rustc_middle::ty::PredicateS<'tcx>,
+            [] consts: rustc_middle::ty::ConstS<'tcx>,
 
             // Note that this deliberately duplicates items in the `rustc_hir::arena`,
             // since we need to allocate this type on both the `rustc_hir` arena