add `str` function for `rune`

examples/rune.v

@@ -0,0 +1,9 @@
+fn main() {
+	// GRINNING FACE😀 => f0 09 98 80
+	grinning_face := rune(0xf09f9880)
+	println(grinning_face)
+
+	// COMMERCIAL AT@ => 0x40
+	commercial_at := rune(0x40000000)
+	println(commercial_at)
+}

vlib/builtin/int.v

@@ -145,10 +145,20 @@ pub fn (a []byte) contains(val byte) bool {
 	return false
 }
 
-/*  TODO 
-fn (c rune) str() string {
+pub fn (c rune) str() string {
+	fst_byte := int(c) >> 8 * 3 & 0xff
+	len := utf8_char_len(fst_byte)
+	mut str := string {
+		len: len
+		str: malloc(len + 1)
 	}
-*/
+	for i := 0; i < len; i++ {
+		str.str[i] = int(c) >> 8 * (3 - i) & 0xff
+	}
+	str[len] = `\0`
+	return str
+}
+
 pub fn (c byte) str() string {
 	mut str := string {
 		len: 1