* guix-kernel/jupyter-client.scm (<notebook>): New record type.
(json->notebook, close-notebook): New procedures.
* guix-jupyter-kernel.scm (%notebook): New variable.
(notebook-info, get-notebook-info-atom)
(notebook-info-control-port, notebook-info-shell-port)
(notebook-info-transport, notebook-info-signature-scheme)
(notebook-info-stdin-port, notebook-info-heartbeat-port)
(notebook-info-ip, notebook-info-iopub-port)
(notebook-info-key, create-address, context)
(addr-heartbeat, addr-shell, addr-control, addr-iopub)
(addr-stdin, socket-heartbeat, socket-shell, socket-control)
(socket-iopub, socket-stdin, addresses, sockets): Remove.
Remove top-level calls to 'zmq-set-socket-option' and
'zmq-bind-socket'.
(proxy-exec-container, heartbeat-handler, general-handler)
(reply-kernel-info-request, reply-execute-request)
(start-container, start-kernel): Refer to %NOTEBOOK instead of referring
to the individual variables.
(atexit, sig-exit-handler): Remove.
(shutdown): Remove call to 'atexit'.
(exit-handler): New procedure.
Use it in top-level 'sigaction' calls.