all: Rename mp_obj_type_t::stream_p to protocol.

It's now used for more than just stream protocol (e.g. pin protocol), so
don't use false names.
diff --git a/py/modio.c b/py/modio.c
index 2fbe6bc..f8826c7 100644
--- a/py/modio.c
+++ b/py/modio.c
@@ -124,7 +124,7 @@
     { &mp_type_type },
     .name = MP_QSTR_BufferedWriter,
     .make_new = bufwriter_make_new,
-    .stream_p = &bufwriter_stream_p,
+    .protocol = &bufwriter_stream_p,
     .locals_dict = (mp_obj_t)&bufwriter_locals_dict,
 };
 #endif // MICROPY_PY_IO_BUFFEREDWRITER