Add new helper macros for minor modes to restore variables

* doc/lispref/modes.texi (Defining Minor Modes): Document it.

* lisp/emacs-lisp/easy-mmode.el (buffer-local-set-state): New macro.
(buffer-local-set-state--get): Helper function.
(buffer-local-restore-state): New function.

* lisp/textmodes/word-wrap-mode.el (word-wrap-whitespace-mode):
Use it to simplify code.

test/lisp/emacs-lisp/easy-mmode-tests.el

@@ -60,6 +60,16 @@
     (easy-mmode-test-mode 'toggle)
     (should (eq easy-mmode-test-mode t))))
 
-(provide 'easy-mmode-tests)
+(ert-deftest test-local-set-state ()
+  (setq global 1)
+  (with-temp-buffer
+    (setq-local local 2)
+    (let ((state (buffer-local-set-state global 10
+                                         local 20
+                                         unexist 30)))
+      (buffer-local-restore-state state)
+      (should (= global 1))
+      (should (= local 2))
+      (should-not (boundp 'unexist)))))
 
 ;;; easy-mmode-tests.el ends here