Array is not traced when sening a msg

```
class Agent {
  def init(arr:[int]) : void {
    ();
  }

}

class Main {
  def main() : void {
    let
      l = 3*1000
      arr = new [int](l)
    in {
      new Agent(arr)
    }
  }
}
```

```
  pony_gc_send();
  /* Not tracing the future in a one_way send */;
  /* Not tracing '_arg_9->f1' */; 
  pony_send_done();
```

As I understand the emitted code, `_arg_9->f1` should be traced.
