* Deprecated functions.php and moved all of those methods to
  ``GuzzleHttp\Streams\Utils``. Use ``GuzzleHttp\Stream\Stream::factory()``
  instead of ``GuzzleHttp\Stream\create()`` to create new streams.
* Added ``flush()`` to ``StreamInterface``. This method is used to flush any
  underlying stream write buffers.
* Added ``FnStream`` to easily decorate stream behavior with callables.
* ``Utils::hash`` now throws an exception when the stream cannot seek to 0.