Logically this function counts how many elements would remain in the alist were we to shrink it with fast-alist-fork.
Good discipline requires that the alist is a fast alist. Under the hood,
when the alist is a fast alist we can simply call the underlying Common Lisp
(defun fast-alist-len (alist) (declare (xargs :guard t)) (len (fast-alist-fork alist nil)))