Disallow object-pointers in process and message slots

[ecpeterson]

In order to ensure program correctness, we need to be very consistent about not letting the author of software-to-be-emulated communicate between objects by any means other than message-passing. A good first approximation to this is to type all message slots, to prevent someone from accidentally passing a (mutable) object. A much better solution is to implement a serialization layer, which will also allow us to gather network load statistics.