This enhances the serialization support to represent Enum values and

CombinedExpression instances.
Enums are stored as a dictionary with metadata needed to resolve the

enum.
CombinedExpression is used only in generated Python code. It's turned

into a <left> <op> <right> expression in code. Support is partially

implemented for versions prior to Django 2.0, but not tested. This

feature is really only used in a way that matters for evolutions in 2.0

or higher, which is important, because prior versions don't support

object deconstruction, meaning they can't be serialized without explicit

serializers written for them.

Tested with upcoming changes that use these types.