This is in part a followup to 81c3dc3224.
* guix/ci.scm (<build>, <checkout>, <evaluation>): Define using
'define-json-mapping'.
(json->build, json->checkout, json->evaluation): Remove.
(queued-builds, latest-builds, latest-evaluations): Pass JSON arrays
through 'vector->list' to adjust for Guile-JSON 3.x.
(evaluations-for-commit): Fix typo to really export.
* guix/ci.scm (<checkout>, <evaluation>): New record types.
(latest-builds): Add #:evaluation and #:system and honor it. Define
'option'.
(json->checkout, json->evaluation, latest-evaluations)
(evaluations-for-commit): New procedures.
