Implement a callback interface for Python functions

[shoaibkamil]

If specializers need to call back into Python, we should provide a standard interface that does it in a serialized manner. The idea is that, when entering the C++ world, a thread gets spawned off and all that thread does is watch a global queue for requests; other threads can put things on that queue and read the returned values from another queue.

Est time: O(1-3 weeks).