allocate intermediate directories in module hierarchy

--- a/src/Tools/Code/code_haskell.ML	Fri Dec 17 16:55:27 2010 +0100
+++ b/src/Tools/Code/code_haskell.ML	Fri Dec 17 18:24:44 2010 +0100
@@ -353,7 +353,7 @@
             val _ = File.check destination;
             val filepath = (Path.append destination o Path.ext "hs" o Path.explode o implode
               o separate "/" o Long_Name.explode) module_name;
-            val _ = Isabelle_System.mkdir (Path.dir filepath);
+            val _ = Isabelle_System.mkdirs (Path.dir filepath);
           in
             (File.write filepath o format [] width o Pretty.chunks2)
               [str "{-# LANGUAGE ScopedTypeVariables #-}", content]