Add field serialization/deserialization support to KeyValueForm.
Review Request #8261 — Created June 29, 2016 and submitted
This adds support for defining serializers/deserializers for fields in KeyValueForm, allowing a form field that returns a non-JSON-serializable object to be stored serialized, and converting it back to the object on load. This is most helpful when using a complex field that returns objects that need special serialization, such as database models.
Unit tests pass.