* lisp/gnus/gnus-sum.el: Use lexical-binding

* lisp/gnus/gnus-sum.el: Use lexical-binding

(gnus-summary-make-menu-bar, gnus-summary-display-make-predicate)
(gnus-summary-refer-thread, gnus-summary-find-matching)
(gnus-summary-edit-article, gnus-summary-sort):
Replace backquoted lambda with closure.
(gnus-summary-article-header): Use define-inline rather than defmacro,
so it's also a function.
(gnus-save-hidden-threads, gnus-summary-iterate, gnus-with-article):
Use `declare`.
(gnus-thread-sort-by-random): Simplify.
(gnus-summary-display-article, gnus-summary-limit-to-address):
Hoist common code outside of `if`.