py/obj: Add "full" and "empty" non-variable-length mp_obj_type_t.

This will always have the maximum/minimum size of a mp_obj_type_t
representation and can be used as a member in other structs.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/modnetwork.h

@@ -62,7 +62,8 @@ mp_obj_t mod_network_nic_ifconfig(struct netif *netif, size_t n_args, const mp_o
 struct _mod_network_socket_obj_t;
 
 typedef struct _mod_network_nic_type_t {
-    mp_obj_type_t base;
+    // Ensure that this struct is big enough to hold any type size.
+    mp_obj_full_type_t base;
 
     // API for non-socket operations
     int (*gethostbyname)(mp_obj_t nic, const char *name, mp_uint_t len, uint8_t *ip_out);