JSONAPI+Testing, JSONAPI+Arbitrary, JSONAPI+OpenAPI

Breaking: Renamed JSONAPITestLib to JSONAPITesting.

Introducing two new framework targets: JSONAPIArbitrary and JSONAPIOpenAPI. Neither is complete, but both offer interesting glimpses into the future. The Arbitrary framework also already does offer utility if you intend to make your models conform to SwiftCheck's Arbitrary protocol for the purpose of generating random values for tests.