Add accessors with reference semantics to objects

New accessor methods are added to composed objects to access to the object
members using the struct. For example, you can access to a sockaddr tipc
portid via sockaddr.port_id() method, and mutate the original sockaddr:

sockaddr sa(portid(1, addr(1, 1, 1));
sa.port_id().ref = 5;

Now sa.addr.id.ref value is 5.