Rework the OO design of S4U comms

Rework the OO design of S4U comms

This sounds much better this way:
-  simgrid::s4u::this_actor::send(mailbox, &pid, comm_size);
+  mailbox->send(&pid, comm_size);

That's intrusive for our (early) users, but the old way should still
work. Only, it raises a deprecation warning.