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)
+}