py: Add hex builtin function.

A one-liner, added especially for @pfalcon :)
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 80cf681..e9f2d8d 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -106,6 +106,8 @@
 Q(getattr)
 Q(globals)
 Q(hash)
+Q(hex)
+Q(%x)
 Q(id)
 Q(io)
 Q(int)