bug#26073: bug in generator function with nested dash anaphoric macros

[Paul Pogonyshev]

To reproduce:

(iter-do (y (funcall (iter-lambda (x)
                       (--each x
                          (let ((y (--map (- it) it)))
                            (iter-yield y))))
                     '((1 2) ( 3 4))))
  (print y))

This should print (-1 -2) and (-3 -4), but instead results in an error.
If you comment out the `iter-yield' form, the example evaluates---
producing different output, of course, but at least without errors.

This looks related to bug #26068, but is not fixed by the patch in
that bug. It is likely related to that both nested dash forms use
`it' variable, and it looks like the wrong (outer) `it' is passed to (- it).

Paul