bug#25154: 25.1; Bindings in cl-letf are in reverse order

[Alex]

Compare the following:

(let ((x 5)
      (x 6))
  (+ x 10))

=> 16

(cl-letf ((x 5)
          (x 6))
  (+ x 10))

=> 15


This also occurs when using non-trivial places:

(setq v (vector 1 2 3 4 5))

(cl-letf (((aref v 1) 10)
          ((aref v 1) 20))
  (aref v 1))

=> 10


I ran into this when using two different setters that sometimes
indirectly modify the same variable. The current behaviour makes the
result of that unexpected.