codegen changes (#2)

* simplify model
- enums are just strings, like in oldschool python. solves forward compatibility issues
- remove _ExplicitlyNullMixin, explicit null is only needed in document types, which support it without this madness
- remove _UnionMixin. Let the service validate it. Might revamp unions anyway

* simpler __eq__

* type hints

* comment public API

Author: graebm