Guile-JSON 3.x is incompatible with Guile-JSON 1.x, which we relied on
until now: it maps JSON dictionaries to alists (instead of hash tables),
and JSON arrays to vectors (instead of lists).  This commit is about
adjusting all the existing code to this new mapping.

* jupyter/kernels.scm (kernel-arguments): Use 'assoc-ref' instead of
'hash-ref'; pass JSON array through 'vector->list'.
(relay-message, reply-html): Likewise.
* jupyter/messages.scm (string->header): Likewise.
* guix-jupyter-container.scm (local-eval, reply-execute-request): Likewise.
* guix-jupyter-kernel.scm (reply-execute-request): Likewise.
* guix-kernel/jupyter-client.scm (json->kernel): Likewise.
* tests/kernels.scm: Likewise.
* environment.scm (propagated-inputs): Replace GUILE-JSON with
GUILE-JSON-3.