factor out transforming AST into OFInput

This will become more complex as new syntax is added and a real intermediate
representation is defined.
diff --git a/Makefile b/Makefile
index 7e9c377..d455ad4 100644
--- a/Makefile
+++ b/Makefile
@@ -79,6 +79,7 @@
 
 check:
 	PYTHONPATH=. ./utest/test_parser.py
+	PYTHONPATH=. ./utest/test_frontend.py
 
 check-py: python
 	PYTHONPATH=${LOXI_OUTPUT_DIR}/pyloxi python py_gen/tests/generic_util.py