use CString and to_bytes_with_nul to be C compatible with \0

[jramapuram]

Works every time now.
Tested by plotting many times at different window resolutions/titles and via:

loop {
    wnd.draw_plot(&af::range(values.dims().unwrap().clone()
                             , 0, af::Aftype::F32).unwrap(), &values, None);
    for i in 0..100 {
      wnd.set_title(i.to_string());
    }
    if wnd.is_closed().unwrap() == true { break; }
  }

[9prady9]

👍