moved default token translations to proof_context.ML, if all fails the pretty printer falls back on plain output;

--- a/src/Pure/Syntax/type_ext.ML	Thu Apr 17 16:30:52 2008 +0200
+++ b/src/Pure/Syntax/type_ext.ML	Thu Apr 17 16:30:53 2008 +0200
@@ -255,7 +255,7 @@
    [],
    [],
    map SynExt.mk_trfun [("fun", K fun_ast_tr')])
-  (map (fn s => ("", s, Output.output_width)) SynExt.standard_token_classes)
+  []
   ([], []);
 
 end;