sum: use variadic functions

sum/sum.go

@@ -9,6 +9,12 @@ func Sum(numbers []int) int {
 	return sum
 }
 
-func SumAll(x []int, y []int) []int {
-	return []int{Sum(x), Sum(y)}
+func SumAll(numbersToSum ...[]int) []int {
+	res := make([]int, len(numbersToSum))
+
+	for i, numbers := range numbersToSum {
+		res[i] = Sum(numbers)
+	}
+
+	return res
 }