Add transformer/serializer/deserializer for Gcstd_eval

[nlevenson]

In Gcapp.write_eval_set_ext, we do a little transformation from Gcstd_eval to JSON (which we ultimately dump as YAML). Similarly, in Gcapp.load_eval_set_ext, we rip a little forEach loop to set all the hashes in our new Gcstd_eval object.

Could this be nicer by moving this activity to the Gcstd_eval class?