PYTHON-1881 added a CFFI binding for libmongocrypt but using this directly in pymongo would be cumbersome because the memory management required (eg mongocrypt_binary_new/mongocrypt_binary_destroy, mongocrypt_new/mongocrypt_init/mongocrypt_destroy, etc...).

We should add a lightweight wrapper around these classes to make managing libmongocrypt objects safe and idiomatic.