Having the `jupyter-comm-layer` abstraction means we do not need to do so.
* jupyter-base.el (zmq): Un-require.
(jupyter-socket-types): Move to `jupyter-channels.el`.
(jupyter-session): Don't mention zmq in doc string.
(jupyter-available-local-ports, jupyter-make-ssh-tunnel): New functions.
(jupyter-tunnel-connection): Use them.
* jupyter-channel-ioloop-comm.el: New file.
* jupyter-channels.el (jupyter-messages): Un-require.
(jupyter-comm-layer, zmq): New requires.
(jupyter-socket-types): Moved from `jupyter-base.el`.
(jupyter-send, jupyter-recv):
Implementations for `jupyter-session` moved from `jupyter-messages.el`.
(jupyter-sync-channel-comm): `jupyter-comm-layer` implementation for
`jupyter-sync-channel` objects moved from `jupyter-comm-layer.el`.
* jupyter-comm-layer.el (jupyter-channel-ioloop): Un-require.
(jupyter-sync-channel-comm): Move implementation to `jupyter-channels.el`.
(jupyter-ioloop-comm): Move implementation to new file `jupyter-ioloop-comm.el`.
(jupyter-channel-ioloop-comm):
Move implementation to new file `jupyter-channel-ioloop-comm.el`.
* jupyter-ioloop-comm.el: New file.
* jupyter-ioloop.el (zmq): Require.
* jupyter-kernel-manager.el
(jupyter-make-client): Ensure `jupyter-channel-ioloop-comm` is required.
* jupyter-messages.el (jupyter-send)
(jupyter-recv): Moved to `jupyter-channels.el`
* jupyter-repl.el
(jupyter-connect-repl): Ensure `jupyter-channel-ioloop-comm` is required.
* test/jupyter-test.el (jupyter-available-local-ports): New test.
* test/test-helper.el (jupyter-channel-ioloop-comm): New require.
