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.

src/base.rs

@@ -668,7 +668,7 @@ fn is_fat_ptr<'tcx>(fx: &FunctionCx<'_, '_, 'tcx>, ty: Ty<'tcx>) -> bool {
                     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 {

src/constant.rs

@@ -46,7 +46,7 @@ pub(crate) fn check_constants(fx: &mut FunctionCx<'_, '_, '_>) -> bool {
             ConstantKind::Ty(ct) => ct,
             ConstantKind::Val(..) => continue,
         };
-        match const_.val {
+        match const_.val() {
             ConstKind::Value(_) => {}
             ConstKind::Unevaluated(unevaluated) => {
                 if let Err(err) =
@@ -127,7 +127,7 @@ pub(crate) fn codegen_constant<'tcx>(
         ConstantKind::Ty(ct) => ct,
         ConstantKind::Val(val, ty) => return codegen_const_value(fx, val, ty),
     };
-    let const_val = match const_.val {
+    let const_val = match const_.val() {
         ConstKind::Value(const_val) => const_val,
         ConstKind::Unevaluated(ty::Unevaluated { def, substs, promoted })
             if fx.tcx.is_static(def.did) =>
@@ -135,7 +135,7 @@ pub(crate) fn codegen_constant<'tcx>(
             assert!(substs.is_empty());
             assert!(promoted.is_none());
 
-            return codegen_static_ref(fx, def.did, fx.layout_of(const_.ty)).to_cvalue(fx);
+            return codegen_static_ref(fx, def.did, fx.layout_of(const_.ty())).to_cvalue(fx);
         }
         ConstKind::Unevaluated(unevaluated) => {
             match fx.tcx.const_eval_resolve(ParamEnv::reveal_all(), unevaluated, None) {
@@ -152,7 +152,7 @@ pub(crate) fn codegen_constant<'tcx>(
         | ConstKind::Error(_) => unreachable!("{:?}", const_),
     };
 
-    codegen_const_value(fx, const_val, const_.ty)
+    codegen_const_value(fx, const_val, const_.ty())
 }
 
 pub(crate) fn codegen_const_value<'tcx>(
@@ -465,7 +465,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
     match operand {
         Operand::Constant(const_) => match const_.literal {
             ConstantKind::Ty(const_) => {
-                fx.monomorphize(const_).eval(fx.tcx, ParamEnv::reveal_all()).val.try_to_value()
+                fx.monomorphize(const_).eval(fx.tcx, ParamEnv::reveal_all()).val().try_to_value()
             }
             ConstantKind::Val(val, _) => Some(val),
         },