* lisp/gnus/nnheader.el (mail-header-*): Define via cl-defstruct

* lisp/gnus/nnheader.el (mail-header-*): Define via cl-defstruct

This also has the side effect that the accessors are now defined as proper
functions rather than as macros, so they can be passed to `mapcar` etc..

* lisp/gnus/nnheader.el (mail-header-number, mail-header-subject)
(mail-header-from, mail-header-date, mail-header-id)
(mail-header-references, mail-header-chars, mail-header-lines)
(mail-header-xref, mail-header-extra): Define via cl-defstruct.
(mail-header-set-number, mail-header-set-subject)
(mail-header-set-from, mail-header-set-date, mail-header-set-id)
(mail-header-set-message-id, mail-header-set-references)
(mail-header-set-chars, mail-header-set-lines, mail-header-set-xref)
(mail-header-set-extra): Remove, use `setf` instead.  All callers adjusted.

* lisp/gnus/gnus-sum.el (gnus-select-newsgroup)
(gnus-summary-pop-limit, gnus-summary-limit-mark-excluded-as-read)
(gnus-summary-find-matching, gnus-find-matching-articles):
* lisp/gnus/gnus-kill.el (gnus-apply-kill-file-internal, gnus-execute):
* lisp/gnus/gnus-score.el (gnus-score-adaptive):
Eta-reduce, now that mail-header-FIELD are functions.