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/ty/sty.rs

@@ -116,7 +116,7 @@ pub enum TyKind<'tcx> {
     Str,
 
     /// An array with the given length. Written as `[T; N]`.
-    Array(Ty<'tcx>, &'tcx ty::Const<'tcx>),
+    Array(Ty<'tcx>, ty::Const<'tcx>),
 
     /// The pointee of an array slice. Written as `[T]`.
     Slice(Ty<'tcx>),