* guix-kernel/jupyter-server.scm (<kernel>): New record type.
(kernel): New procedure.
(run-kernel): Return a <kernel> instead of an alist.
* guix-jupyter-container.scm (register-kernel): Replace 'name' and
'values' parameters with 'kernel'.
(kernel-by-name): Return the kernel itself, not the pair.
(kernel-shell-by-name, kernel-iosub-by-name): Remove.
(proxy-exec-kernel): Adjust accordingly.