Support inductive relations with type parameters #48
Description
Right now, all the inductive relations in our test suite only contain indices -- none of these inductive relations contain (type) parameters. We should extend Chamelean to handle inductive relations with parameters -- it's unclear how to do so at the moment, but as a first step, if we're trying to derive an ArbitrarySuchThat instance for a relation R with type parameter a, then the derived generator should expect there to be an Arbitrary a instance, and invoke it when appropriate.