Send the header metadata with a message instead of ignoring the header
metadata and sending "{}".

The header metadata is handled correctly on receipt of a message. Why
was it ignored on sending? Probably no-one ever needed it.

The Wire Protocol allows for "extra raw data buffer(s)" at the end of a
ZeroMQ message. This commit enables buffers in the ipython-kernel.

https://jupyter-client.readthedocs.io/en/stable/messaging.html#the-wire-protocol

This field has been in "the Wire Protocol" since before the Jupyter Message
specification version 5.0.

https://github.com/jupyter/jupyter_client/issues/520

Resolves issue

https://github.com/gibiansky/IHaskell/issues/1144

I've tested this feature with a proprietary JupyterLab extension and
I've verified that it works. It's difficult to provide a public
reproducible test. The best test may be to get ipywidgets Image working--
that uses buffers.

instead of hardcoded localhost (127.0.0.1)

This commit adds new message update_dispaly_data and updates
display_data message to add optional transient field which
is in turn used by update_display_data

The IPython specification for the metadata does not
define that all fields in metadata must be textual.
Latest Jupyters lab uses that feature so the parsing
mechanism became incomplatible with the protocol.
We change Metadata to be an Aeson.Object and introduce
a 'Metadata' newtype in order to keep that explicit
and open for the later changes if needed.

Fixes #912.

Fixing these warnings required changes to the file names of the `MessageHeader`
struct which makes this commit an API change. The version in the cabal file
has been bumped accordingly.