Support bidirectional migrations #8

At the moment, data can be migrated only from older to newer versions, but in some circumstances we might want the opposite. This would require some thought as the existing changelog format does not include enough information to go backwards (e.g. when a type is removed, we do not record its definition). We need to find the right trade-off between extending the changelog with additional information, supplying appropriate defaults, and warning the user when a backwards migration may lead to invalid data.