This avoids the occasional string decoding error for byte streams that
were not actual UTF-8 strings.

This is based on commit fbab29d738b46d8637579ccec93be49520a4606f ("Fixed
a bug when the kernel could not send kernel_info (1 minute pause after
notebook starts)") of <https://github.com/jerry40/guile-kernel

>.

* guix-jupyter-container.scm (reply-execute-request): Turn UUID into a
bytevector.
* guix-jupyter-kernel.scm (send-to-container): Turn NAME into a
bytevector and use 'zmq-send-msg-parts-bytevector'.
(heartbeat-handler): Use '-bytevector' send and receive procedures.
(general-handler): Likewise.  Add calls to 'utf8->string' as needed.
* guix-kernel/jupyter-client.scm (send-to-jupyter)
(pub): Likewise.
* guix-kernel/jupyter-server.scm (run-kernel): Likewise.

Co-authored-by: jerry40 <epanfilov@gmail.com>