This change introduces a Json format change where we always print fully
qualified type names everywhere. This is beneficial for diagnosing purposes but
may not be ideal for generating migrator scripts. To resolve this conflict, the
patch also introduces a flag -migrator to opt-out fully qualified type names.
After the format change, we need to update the ABI and API baselines for the
Swift stdlib.
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.
