Changed future_result and try to return Result<(), ~Any>.

- Internally, there is still an enum passend around.
- In case of linked failure or a string message, the ~Any gets
  lazyly allocated in future_results recv method.
- For that, future result now returns a wrapper around a Port.
- Moved task::TaskResult into rt::task and made it an internal enum.
- Introduced an replacement typedef `type TaskResult = Result<(), ~Any>`.