Because our builder append methods are not cdef functions, Cython is creating Python wrapper objects to pass to the method, and then converting those Python objects back to C object when calling the underlying append method of the Arrow builder class.   We can avoid this overhead by adding a lower-level function to each builder class that is used from within process_bson_stream, and pass C objects directly to the Arrow builders where possible.

This should greatly improve our speed and memory peformance.