Message Bus for Stratego4

## Overview

This is an Epic encapsulating research on using an EVM as a message bus and then applying that concept to the game of Stratego4.

Stratego4 was chosen as a simple example of cross-company blockchain patterns:
1. Information that all parties know and share
2. Information that each party keeps secret to themselves
3. Information shared between a subset of the parties
4. Shared information that can eventually be proven true or false

### Reference

- [Atomic Crosschain Transactions (Pantheon)](https://arxiv.org/abs/1904.12079)
- [chain.link](https://docs.chain.link/docs)
- [Original Stratego4 Rules](https://github.com/ConsenSys/web3studio-sidejam/issues/1#issuecomment-490923699)
- [General Message Bus Pattern](https://github.com/ConsenSys/web3studio-sidejam/issues/1#issuecomment-490906621)
- [Tx Flow for Attacking](https://github.com/ConsenSys/web3studio-sidejam/issues/1#issuecomment-490916148)
- [Gameplay storyboard](https://github.com/ConsenSys/web3studio-sidejam/issues/1#issuecomment-490902470)

### Assumptions

- Even Stratego4's rules can be complex, the rules can and likely should be modified to produce a simpler, better, metaphor
- Using multiple chains would be cool, for the first version, public-key encryption will be easier and should be used
- Preventing cross-chain double spend is a separate problem than a message bus. They should be thought of as separate problems (message bus + plasma), not as one solution. There isn't anything special we need to do in order to not complect these two ideas.

### Acceptance

- Documented architecture pattern for a Message Bus on an EVM
- Should help to prevent deadlocks
- Should not get in the way of the existing cross-chain double spend solutions (see assumptions)
- Should attempt to document failure states
- A Playable Stratego4 using a message bus