Actors may be destroyed without ever calling their receivers. This is
regular behavior when an actor is killed by an exit message. C++ actors
cleanup automatically via their destructors. Up to now zig actor had to
enable trapping and cleanup on the exit message. This was a big foot gun
and cumbersome.
Now all zig actors are required to pass a deinit function to
thespian.receive. This simplifies clean up and prevents the foot gun
entirely.
