registry.el (registry-reindex): New method to recreate the secondary registry indices.

registry.el (registry-reindex): New method to recreate the secondary registry indices.
gnus-registry.el (gnus-registry-fixup-registry): Use it if the tracked field changes.
 (gnus-registry-unfollowed-addresses, gnus-registry-track-extra)
 (gnus-registry-action, gnus-registry-spool-action)
 (gnus-registry-handle-action)
 (gnus-registry--split-fancy-with-parent-internal)
 (gnus-registry-split-fancy-with-parent)
 (gnus-registry-register-message-ids): Add recipient tracking on spool, move, and delete actions, and for fancy splitting with parent.
 (gnus-registry-extract-addresses)
 (gnus-registry-fetch-recipients-fast)
 (gnus-registry-fetch-header-fast): Convenience functions.
 (gnus-registry-misc-test): ERT test of `gnus-registry-extract-addresses'.