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_codegen_cranelift/src/base.rs

@@ -668,7 +668,7 @@ fn codegen_stmt<'tcx>(
                     let times = fx
                         .monomorphize(times)
                         .eval(fx.tcx, ParamEnv::reveal_all())
-                        .val
+                        .val()
                         .try_to_bits(fx.tcx.data_layout.pointer_size)
                         .unwrap();
                     if operand.layout().size.bytes() == 0 {