nth function added to stdlib, doc updates

stdlib/stdlib.lsp

@@ -49,6 +49,9 @@
 ; return fifth element of list
 (defun fifth (l) (index l 4))
 
+; return nth element of list, indexing from 0
+(defun nth (i l) (index l i))
+
 ; return 1 when list contains item otherwise 0
 (defun member (lst itm) 
     (do