remove reference counting headers from ~

Unique pointers and vectors currently contain a reference counting
header when containing a managed pointer.

This `{ ref_count, type_desc, prev, next }` header is not necessary and
not a sensible foundation for tracing. It adds needless complexity to
library code and is responsible for breakage in places where the branch
has been left out.

The `borrow_offset` field can now be removed from `TyDesc` along with
the associated handling in the compiler.

Closes #9510
Closes #11533

src/libstd/rc.rs

@@ -230,4 +230,12 @@ mod tests {
         drop(x);
         assert!(y.upgrade().is_none());
     }
+
+    #[test]
+    fn gc_inside() {
+        // see issue #11532
+        use gc::Gc;
+        let a = Rc::new(RefCell::new(Gc::new(1)));
+        assert!(a.borrow().try_borrow_mut().is_some());
+    }
 }