added Reduce method

reduce.go

@@ -0,0 +1,15 @@
+package slices
+
+import "git.milar.in/milarin/gmath"
+
+func Reduce[T, R any](slice []T, reducer func(current R, v T) R) R {
+	res := new(R)
+	Each(slice, func(v T) {
+		*res = reducer(*res, v)
+	})
+	return *res
+}
+
+func SumReducer[N gmath.Number](a, b N) N {
+	return a + b
+}