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_const_eval/src/interpret/intrinsics/type_name.rs

@@ -68,7 +68,7 @@ impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
         }
     }
 
-    fn print_const(self, ct: &'tcx ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
+    fn print_const(self, ct: ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
         self.pretty_print_const(ct, false)
     }
 

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -561,10 +561,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// "universe" (param_env).
     pub fn const_to_op(
         &self,
-        val: &ty::Const<'tcx>,
+        val: ty::Const<'tcx>,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
-        match val.val {
+        match val.val() {
             ty::ConstKind::Param(_) | ty::ConstKind::Bound(..) => throw_inval!(TooGeneric),
             ty::ConstKind::Error(_) => throw_inval!(AlreadyReported(ErrorReported)),
             ty::ConstKind::Unevaluated(uv) => {
@@ -574,7 +574,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             ty::ConstKind::Infer(..) | ty::ConstKind::Placeholder(..) => {
                 span_bug!(self.cur_span(), "const_to_op: Unexpected ConstKind {:?}", val)
             }
-            ty::ConstKind::Value(val_val) => self.const_val_to_op(val_val, val.ty, layout),
+            ty::ConstKind::Value(val_val) => self.const_val_to_op(val_val, val.ty(), layout),
         }
     }
 
@@ -584,7 +584,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
         match val {
-            mir::ConstantKind::Ty(ct) => self.const_to_op(ct, layout),
+            mir::ConstantKind::Ty(ct) => self.const_to_op(*ct, layout),
             mir::ConstantKind::Val(val, ty) => self.const_val_to_op(*val, *ty, layout),
         }
     }

compiler/rustc_const_eval/src/interpret/util.rs

@@ -55,7 +55,7 @@ where
                                     assert!(matches!(ty.kind(), ty::Param(_)))
                                 }
                                 ty::subst::GenericArgKind::Const(ct) => {
-                                    assert!(matches!(ct.val, ty::ConstKind::Param(_)))
+                                    assert!(matches!(ct.val(), ty::ConstKind::Param(_)))
                                 }
                                 ty::subst::GenericArgKind::Lifetime(..) => (),
                             },
@@ -68,8 +68,8 @@ where
             }
         }
 
-        fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
-            match c.val {
+        fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+            match c.val() {
                 ty::ConstKind::Param(..) => ControlFlow::Break(FoundParam),
                 _ => c.super_visit_with(self),
             }