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_middle/src/mir/visit.rs

@@ -200,7 +200,7 @@ macro_rules! make_mir_visitor {
             }
 
             fn visit_const(&mut self,
-                           constant: & $($mutability)? &'tcx ty::Const<'tcx>,
+                           constant: $(& $mutability)? ty::Const<'tcx>,
                            _: Location) {
                 self.super_const(constant);
             }
@@ -864,7 +864,7 @@ macro_rules! make_mir_visitor {
                 self.visit_span(span);
                 drop(user_ty); // no visit method for this
                 match literal {
-                    ConstantKind::Ty(ct) => self.visit_const(ct, location),
+                    ConstantKind::Ty(ct) => self.visit_const($(& $mutability)? *ct, location),
                     ConstantKind::Val(_, ty) => self.visit_ty($(& $mutability)? *ty, TyContext::Location(location)),
                 }
             }
@@ -903,7 +903,7 @@ macro_rules! make_mir_visitor {
             fn super_region(&mut self, _region: $(& $mutability)? ty::Region<'tcx>) {
             }
 
-            fn super_const(&mut self, _const: & $($mutability)? &'tcx ty::Const<'tcx>) {
+            fn super_const(&mut self, _const: $(& $mutability)? ty::Const<'tcx>) {
             }
 
             fn super_substs(&mut self, _substs: & $($mutability)? SubstsRef<'tcx>) {