py: Replace stream_p with *stream_p in mp_obj_type_t.

This is to reduce ROM usage.  stream_p is used in file and socket types
only (at the moment), so seems a good idea to make the protocol
functions a pointer instead of the actual structure.

It saves 308 bytes of ROM in the stmhal/ port, 928 in unix/.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 2271941..85f6986 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -111,6 +111,7 @@
 Q(min)
 Q(namedtuple)
 Q(next)
+Q(open)
 Q(ord)
 Q(path)
 Q(pow)