It would be useful from pymongocrypt to abstract away libmongocrypt's state machine and provide a simple callback api for pymongo to consume. Pymongo then only needs to implement the I/O callbacks (fetch data key from the key vault, run listCollections, etc..).

My goals are that:

1. pymongo does not need to call any libmongocrypt CFFI methods directly.
2. pymongocrypt does not need to depend on pymongo so that there is no circular dependency