* jupyter/json.scm (json, <=>): New variables.
(define-json-reader): Adjust to the new syntax for SPEC.
(define-json-writer): Likewise.
(define-json-mapping): Rebase on top of 'define-record-type*'.  Move
JSON field spec under the 'json' keyword.
* jupyter/kernels.scm (<kernel-specs>, <connection>): Adjust
accordingly.
(find-kernel-specs, allocate-connection): Use new syntactic
constructors.