* jupyter/kernels.scm (generate-key): New procedure.
(relay-message): Remove.
* jupyter/messages.scm (<message>): Use 'define-immutable-record-type'
and define 'set-message-content'.
* guix-jupyter-container.scm (local-eval): Rewrite in terms of
'send-message'.
(reply-html-to-kernel): Remove.
(general-handler): Copy from *-kernel.scm.
(execute-request-sans-magic): New procedure.
(ignore-request, reply-execute-request, shutdown): Adjust to new calling
convention.
(%main-kernel, %main-kernel-id): New variables.
<top level>: Use 'serve-kernels'.
* guix-jupyter-kernel.scm (%main-kernel, %main-kernel-id): New
variables.
(general-handler): Check whether we're talking to %MAIN-KERNEL.
(reply-kernel-info-request, reply-execute-request, shutdown): Add 'kind'
parameter.
(new-container-connect): Remove.
(start-container): Return a <kernel>.
<top level>: Pass both the "default" container kernel and our client to
'server-kernels'.  Set %MAIN-KERNEL.