Summary:
A specialization for reading into enums from serialized form was added a while back. Unfortunately, no corresponding change to serialize from enums into a dynamic was added.

This change adds the corresponding DynamicConstructor specialization for enums to allow serializing enums and objects containing enums.

Reviewed By: ot, shixiao

Differential Revision: D14091417

fbshipit-source-id: 15ef8bc46843cd668b4b2c991e84167dabaf69fa