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/librustdoc/clean/utils.rs

@@ -226,8 +226,8 @@ crate fn name_from_pat(p: &hir::Pat<'_>) -> Symbol {
     })
 }
 
-crate fn print_const(cx: &DocContext<'_>, n: &ty::Const<'_>) -> String {
-    match n.val {
+crate fn print_const(cx: &DocContext<'_>, n: ty::Const<'_>) -> String {
+    match n.val() {
         ty::ConstKind::Unevaluated(ty::Unevaluated { def, substs: _, promoted }) => {
             let mut s = if let Some(def) = def.as_local() {
                 let hir_id = cx.tcx.hir().local_def_id_to_hir_id(def.did);
@@ -297,15 +297,15 @@ fn format_integer_with_underscore_sep(num: &str) -> String {
         .collect()
 }
 
-fn print_const_with_custom_print_scalar(tcx: TyCtxt<'_>, ct: &ty::Const<'_>) -> String {
+fn print_const_with_custom_print_scalar(tcx: TyCtxt<'_>, ct: ty::Const<'_>) -> String {
     // Use a slightly different format for integer types which always shows the actual value.
     // For all other types, fallback to the original `pretty_print_const`.
-    match (ct.val, ct.ty.kind()) {
+    match (ct.val(), ct.ty().kind()) {
         (ty::ConstKind::Value(ConstValue::Scalar(int)), ty::Uint(ui)) => {
             format!("{}{}", format_integer_with_underscore_sep(&int.to_string()), ui.name_str())
         }
         (ty::ConstKind::Value(ConstValue::Scalar(int)), ty::Int(i)) => {
-            let ty = tcx.lift(ct.ty).unwrap();
+            let ty = tcx.lift(ct.ty()).unwrap();
             let size = tcx.layout_of(ty::ParamEnv::empty().and(ty)).unwrap().size;
             let data = int.assert_bits(size);
             let sign_extended_data = size.sign_extend(data) as i128;