incorporate information about initial, immovable heap objects

[davepacheco]

See nodejs/post-mortem#50 (comment) for details. It sounds like with a small amount of additional metadata (fixed addresses of initial objects), we can potentially get access to more information about the layout of the heap and different types of objects. I'm not sure yet what we can do with this, but it's worth exploring. We may be able to do more directed traversals of the JS heap (if we have information about top-level heap structures) or construct a list of types without having traversed the entire heap.