cleanup: replace C for loops with range

vlib/builtin/bare/array_bare.v

@@ -47,7 +47,7 @@ pub fn (a array) repeat(nr_repeats int) array {
 		element_size: a.element_size
 		data: malloc(nr_repeats * a.len * a.element_size)
 	}
-	for i := 0; i < nr_repeats; i++ {
+	for i in 0..nr_repeats {
 		mem_copy(arr.data + i * a.len * a.element_size, a.data, a.len * a.element_size)
 	}
 	return arr

vlib/builtin/bare/string_bare.v

@@ -28,10 +28,10 @@ fn (s string) add(a string) string {
 		len: new_len
 		str: malloc(new_len + 1)
 	}
-	for j := 0; j < s.len; j++ {
+	for j in 0..s.len {
 		res[j] = s[j]
 	}
-	for j := 0; j < a.len; j++ {
+	for j in 0..a.len {
 		res[s.len + j] = a[j]
 	}
 	res[new_len] = `\0`// V strings are not null terminated, but just in case