utf8: format utf8_util.v (#10135)

vlib/encoding/utf8/utf8_util.v

@@ -113,13 +113,13 @@ pub fn raw_index(s string, index int) string {
 // reverse - returns a reversed string.
 // example: utf8.reverse('你好世界hello world') => 'dlrow olleh界世好你'.
 pub fn reverse(s string) string {
-	len_s := utf8.len(s)
+	len_s := len(s)
 	if len_s == 0 || len_s == 1 {
 		return s.clone()
 	}
 	mut str_array := []string{}
-	for i in 0..len_s {
-		str_array << utf8.raw_index(s, i)
+	for i in 0 .. len_s {
+		str_array << raw_index(s, i)
 	}
 	str_array = str_array.reverse()
 	return str_array.join('')