* lisp/gnus: Further reduce assumptions about gnus-data format

* lisp/gnus: Further reduce assumptions about gnus-data format

* lisp/gnus/gnus-registry.el (gnus-registry-fetch-message-id-fast)
(gnus-registry-fetch-simplified-message-subject-fast)
(gnus-registry-fetch-sender-fast, gnus-registry-fetch-recipients-fast)
(gnus-registry--set/remove-mark): Don't hardcode assoc for gnus-data-find-in.

* lisp/gnus/gnus-sum.el (gnus-data-update-list): Don't hardcode `nth 2`
for gnus-data-pos.
(gnus-summary-insert-old-articles, gnus-summary-insert-new-articles)
(gnus-summary-first-article-p): Don't hardcode `car` for `gnus-data-number`.
(gnus-summary-move-article, gnus-summary-expire-articles)
(gnus-summary-delete-article): Don't hardcode assoc for gnus-data-find-in.

* lisp/gnus/spam.el (spam-fetch-article-header): Don't hardcode `nth 3`
for gnus-data-header.