py: Add very basic implementation of dir() builtin.

Only works on modules and class instances.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 8bf3936..592b849 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -51,6 +51,7 @@
 Q(chr)
 Q(complex)
 Q(dict)
+Q(dir)
 Q(divmod)
 Q(enumerate)
 Q(eval)