sim: use Simulator as a contextmanager

__del__ garbage collector callbacks are too delicate.  E.g. imported
modules can be garbage collected before the objects using them. Can't
use os.remove, socket.SHUT_RDWR...

* added a DeprecationWarning if a Simulator is garbage collected without
having its .close() called
* renamed all gc __del__ callbacks to close()
* implemented context manager hooks for Simulator. Use like

   with Simulator(TestBench()) as s:
       s.run()