This introduces Certificate.to_json(), which creates a serialized

representation of the certificate information. This will be useful for

API payloads soon for error results.
CertificateFingerprints.serialize() has been renamed to to_json(),

allowing it to be included in a SerializableJSONValue payload.

deserialize() was renamed to from_json() for consistency. Going

forward, we should strive for these names for any JSON serialization

functionality.
CertificateFingerprints also gained an equality check, needed for the

Certificate.to_json() tests.

Unit tests pass.
Made use of to_json() in an upcoming change for API errors.