This item is to associate a usr with a special case Id, however, it's
up to the migrator to interpret the special case Id and apply proper
transformations.
This structure serves multiple purposes:
a. Allow migrator to speak in the same language with swift-api-digester in terms of API changes.
b. Serialize/Deserialize detected API change items in JSON format.
c. Manage memory after deserializing API change items.
d. Facilitate look up by USRs of APIs under migration.
The structure is tested by round-trip serialization and deserialization.
