Improve support for enum_types

Reviewer: @rlane @vaterlaus

This PR improves support for enum types in loxi. 

The first three commits add support to the infratstructure, add code to handle (read: ignore) the enum values in c_loxi and py_loxi. @Rich: can you validate those?

The last three commits add the new support in java_gen. @RobV, please validate.