Change description serialization has been refactored into three

methods:

serialization of lists of models,
serialization of lists of plain old data, and
serialization of singletons (which are stored as a list with a single

  element in the ChangeDescription's fields_fields field).

This allows for code de-duplication and consolidation for the

BaseReviewRequestField and BaseCommaEditableField. The logic for

serialization has also been corrected to only attempt to serialize

added and removed entries when they exist.
The serialization of lists of models has also been corrected.

Previously, there were no BaseReviewRequestField instances that

serialized lists of models. If there were, they would have thrown an

exception because the default implementation attempted to serialize

them from a tuple not wrapped within a list.

Ran unit tests.
Used this with an upcoming change; it worked correctly.