Make fallback API match Cython's for attributes #44
Description
Currently fallback.Unpacker has a list_hook attribute publicly available. It would not be surprising if a user decided to change the value of list_hook after the Unpacker was constructed, but such functionality is not compatible with the Cython version.
Hence, we should probably preface the attributes of Unpacker and Packer with _ to make it clear that they are not to be used without caution.