Trait for creating the ArrTBuilder and ArrTFlatBuilder type class instances for ArrDblNs final classes. Instances for the ArrBuilder type
class, for classes / traits you control, should go in the companion object of B. Instances for [[ArrFlatBuilder] should go in the companion
object the ArrT final class. The first type parameter is called B, because to corresponds to the B in
map(f: A => B): ArrB function.