Completely use cl-lib and lexical-binding

Previously both cl-lib.el and cl.el were used, now use only cl-lib.el.
Use lexical-binding where needed instead of requiring cl just for
`lexical-let`.
Replace some add-to-list with cl-pushnew as add-to-list is not
recommended in lisp program and anyway doesn't work properly with
lexical binding.

mu4e/org-mu4e.el

@@ -197,9 +197,10 @@ and images in a multipart/related part."
                        url (file-name-directory current-file)))
                 (ext (file-name-extension path))
                 (id (replace-regexp-in-string "[\/\\\\]" "_" path)))
-           (add-to-list 'html-images
-                        (org~mu4e-mime-file
-        (concat "image/" ext) path id))
+           (cl-pushnew (org~mu4e-mime-file
+                         (concat "image/" ext) path id)
+                       html-images
+                       :test 'equal)
            id)))
       str)
      html-images)))