Programming API for Atomic Message Passing in Heterogeneous Paxos Chains

[isheff]

Implement a programming model for atomic message passing between state machines running on a Heterogeneous Paxos Chain. This can be based on IBC, bug should provide additional atomicity guarantees. Note that:

• We need to communicate to the programmer exactly what the atomicity guarantees are
• Not all nodes understand all state machines, so one node needs to prove that a message was sent with some specific content, so that the other nodes can progress, having read the message.