RefCell<Option<Heap<T>>> is easy to misuse

Code like this is subtly incorrect:

        // Cache the Js value.
        let heap_val = Heap::default();
        heap_val.set(frozen_types);
        *self.frozen_supported_performance_entry_types.borrow_mut() = Some(heap_val);

Moving around Heap values invalidates pointers; the correct way to use this is to set the Some(Heap::default()), then use borrow_mut().as_ref().unwrap().set(...) to update the value after it has settled in memory.