Add serialization of evaluator objects

[nsmith-]

At the moment, evaluator Correction objects are read-only and can only be de-serialized from the JSON form. If they could also be serialized, then this would improve the performance of pickling the python wrapper, which currently stores a copy of the JSON to be able to re-construct the evaluator objects. It would probably amount to creating inverses of the various node constructors that output rapidjson JSONObjects, and then implementing pickling directly in the pybind11 code https://pybind11.readthedocs.io/en/stable/advanced/classes.html?#pickling-support