Avoid calling length each iteration of for-each

length is O(n) so calling it should be avoided, especially for each iteration of for-each. Instead the code can just check if the first cdr is null. This has the potential for a huge speed improvement.

scheme/base.sld

@@ -764,7 +764,8 @@
                     (apply f cars)
                     (recur cdrs)))))
           ;; Fast path.
-          (if (eq? 1 (length lis1))
+          ;(if (eq? 1 (length lis1))
+          (if (null? (cdr lis1)) ;; O(1) instead of O(n) for length
             (f (car lis1))
             (begin (f (car lis1))
                    (for-each f (cdr lis1)))))))