messages: Add <kernel-status>.

* jupyter/messages.scm (<kernel-status>): New record type.
* jupyter/kernels.scm (pub): Expect a <kernel-status> record and use
'kernel-status->json'.
(pub-busy, pub-idle): Pass a <kernel-status> record.
* tests/kernels.scm ("run-kernel python3")
("kernel_info_request", "execute_request"): Use 'json->kernel-status'
instead of alists.