Trait for creating the ArrTBuilder and ArrTFlatBuilder type class instances for Int2Arr 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 a sub class of Int2Elem, because to corresponds to the B in the
map(f: A => B): ArrB function.