println: optimize and remove memory leaks

compiler/tests/struct_test.v

@@ -8,6 +8,7 @@ struct B{
 mut: 
 	a A 
 } 
+
 struct C {
 mut: 
 	b B 
@@ -15,6 +16,11 @@ mut:
 	as []A 
 	num int 
 } 
+
+struct User {
+	name string
+	age int 
+} 
  
 fn test_struct_levels() { 
 	mut c := C{} 
@@ -42,3 +48,9 @@ fn test_struct_levels() {
 	c.as[0].val  = 10 
 	assert c.as[0].val == 10 
 } 
+
+fn test_struct_str() {
+	u := User{'Bob', 30} 
+	println(u)  // make sure the struct is printable 
+	// assert u.str() == '{name:"Bob", age:30}'  // TODO 
+}