py: Add typecode to buffer protocol.

When querying an object that supports the buffer protocol, that object
must now return a typecode (as per binary.[ch]).  This does not have to
be honoured by the caller, but can be useful for determining element
size.
diff --git a/py/modstruct.c b/py/modstruct.c
index 3b9679a..945f4fe 100644
--- a/py/modstruct.c
+++ b/py/modstruct.c
@@ -55,7 +55,7 @@
     char fmt_type = get_fmt_type(&fmt);
     uint size = calcsize_items(fmt);
     mp_obj_tuple_t *res = mp_obj_new_tuple(size, NULL);
-    buffer_info_t bufinfo;
+    mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(data_in, &bufinfo);
     byte *p = bufinfo.buf;