Switch to Guile-JSON 3.x.

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.

* src/cuirass/http.scm (evaluation->json-object): New procedure.
(handle-builds-request): Pass the result through 'list->vector'.
(handle-builds-search-request): Likewise.
(url-handler): Likewise for /jobsets, /specifications, /api/evaluations,
and /build.  For /api/evaluations, use 'evaluation->json-object'.
* src/cuirass/utils.scm (object->json-scm): Add 'vector?' case.
* tests/http.scm (hash-table-keys, hash-table=?): Remove.
(evaluations-query-result): Use vectors for JSON arrays.
("object->json-string"): Expects alists instead of hash tables.
("/build/1"): Use 'lset=' instead of 'hash-table=?'.
("/api/latestbuilds?nr=1&jobset=guix"): Likewise, and expect alists
instead of hash tables.
("/api/latestbuilds?nr=1&jobset=gnu"): Likewise.
("/api/evaluations?nr=1"): Likewise.
* README: Mention Guile-JSON 3.x.