array: add reduce() method

vlib/builtin/array.v

@@ -354,3 +354,20 @@ pub fn (a []int) filter(predicate fn(p_val int, p_i int, p_arr []int) bool) []in
 	}
 	return res
 }
+
+////////////// REDUCE //////////////
+
+// method executes a reducer function (that you provide) on each element of the array, 
+// resulting in a single output value.
+pub fn (a []int) reduce(
+	iter fn (accum int, curr int) int,
+	accum_start int
+) int
+{
+	mut _accum := 0
+	_accum = accum_start
+	for i := 0; i < a.len; i++ {
+			_accum = iter(_accum, a[i])
+	}
+	return _accum
+}