Versioning

This is a big problem space, but we can try to support at least a little.

For now, CObject treats children like protobuf optional values, i.e., it doesn't complain if it gets a message/savedState for a child it doesn't have. This allows protobuf-style versioning (add and delete CObject children, so long as names stay fixed over time). However, we need to document this.

Is that sufficient for real apps? Or do we need something more Cambria-style?