ArrDbl2sFlatBuilder
Trait for creating the ArrTFlatBuilder type class instances for Dbl2Arr final classes. Instances for [[ArrFlatBuilder] should go in the companion object the ArrT final class. The first type parameter is called B, because it corresponds to the B in
build: ArrTBuilder[B, ArrB]): ArrB
function. build: ArrTBuilder[B, ArrB]): ArrB