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/.
Merge branch 'master' of github.com:micropython/micropython