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_monomorphize/src/polymorphize.rs

@@ -267,7 +267,7 @@ impl<'a, 'tcx> Visitor<'tcx> for MarkUsedGenericParams<'a, 'tcx> {
         self.super_local_decl(local, local_decl);
     }
 
-    fn visit_const(&mut self, c: &&'tcx Const<'tcx>, _: Location) {
+    fn visit_const(&mut self, c: Const<'tcx>, _: Location) {
         c.visit_with(self);
     }
 
@@ -278,12 +278,12 @@ impl<'a, 'tcx> Visitor<'tcx> for MarkUsedGenericParams<'a, 'tcx> {
 
 impl<'a, 'tcx> TypeVisitor<'tcx> for MarkUsedGenericParams<'a, 'tcx> {
     #[instrument(level = "debug", skip(self))]
-    fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+    fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         if !c.has_param_types_or_consts() {
             return ControlFlow::CONTINUE;
         }
 
-        match c.val {
+        match c.val() {
             ty::ConstKind::Param(param) => {
                 debug!(?param);
                 self.unused_parameters.clear(param.index);
@@ -348,12 +348,12 @@ impl<'a, 'tcx> TypeVisitor<'tcx> for HasUsedGenericParams<'a> {
     type BreakTy = ();
 
     #[instrument(level = "debug", skip(self))]
-    fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> ControlFlow<Self::BreakTy> {
+    fn visit_const(&mut self, c: Const<'tcx>) -> ControlFlow<Self::BreakTy> {
         if !c.has_param_types_or_consts() {
             return ControlFlow::CONTINUE;
         }
 
-        match c.val {
+        match c.val() {
             ty::ConstKind::Param(param) => {
                 if self.unused_parameters.contains(param.index).unwrap_or(false) {
                     ControlFlow::CONTINUE