(map1): Rewrite to be tail-recursive.

Thanks to Panagiotis Vossos for the bug report.

srfi/srfi-1.scm

@@ -603,10 +603,15 @@
 ;; Internal helper procedure.  Map `f' over the single list `ls'.
 ;;
 (define (map1 f ls)
-  (let lp ((l ls))
-    (if (null? l)
-      '()
-      (cons (f (car l)) (lp (cdr l))))))
+  (if (null? ls)
+      ls
+      (let ((ret (list (f (car ls)))))
+        (let lp ((ls (cdr ls)) (p ret))         ; tail pointer
+          (if (null? ls)
+              ret
+              (begin
+                (set-cdr! p (list (f (car ls))))
+                (lp (cdr ls) (cdr p))))))))
 
 ;; This `map' is extended from the standard `map'.  It allows argument
 ;; lists of different length, so that the shortest list determines the