[Breaking change] flatten and expand_types now unkey

The PR #4290 changed Table.flatten and Table.expand_types, so that they now always return an unkeyed table. They are otherwise unchanged. Due to implementation details, it is currently significantly more efficient to remove keys, and we felt that for most use cases of these methods removing the keys from the result shouldn’t have any effect. If you do need the result to be keyed, adding an explicit key_by will have the same behavior as the old implementation.