Trait for creating the ArrTBuilder and ArrTFlatBuilder type class instances for Dbl4Arr final classes. Instances for the ArrBuilder type
class, for classes / traits you control, should go in the companion object of type B, which will extend ElemDbl4. 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
map(f: A => B): ArrB function.