* guix-kernel/jupyter-client.scm (<header>, <message>, message)
(make-message, message-parts, reply, message-type, message-sender)
(string->header, header->string, parts->message, DELIM): Move to...
* jupyter/messages.scm: ... here.  New file.
(get-signature, make-id): New procedures.
* guix-kernel/hmac.msc, guix-kernel/tools.scm, tests/tools.scm: Remove.
* Makefile.am (SOURCES, SCM_TESTS): Adjust accordingly.
* guix-jupyter-container.scm, guix-jupyter-kernel.scm,
guix-kernel/jupyter-client.scm: Adjust import list accordingly.
* tests/hmac.scm (get-signature): New variable.