Encapsulate all proto enum types within a message so that they are namespaced in C++/Python

Python/C++ dump enums into the parent namespace. By placing the enum within a containing message, we can provide a meaningful name for enums and avoid collisions in names.

See:
https://developers.google.com/protocol-buffers/docs/cpptutorial#enums-and-nested-classes
https://developers.google.com/protocol-buffers/docs/pythontutorial#enums