Introduce ChallangerAgent and DisputeGame

[kerzhner]

Follows up on https://github.com/statechannels/dispute-game/pull/43. Fixes https://github.com/statechannels/dispute-game/issues/34.

This refactor includes:

• A ChallengerAgent represents a player in the dispute game. The challenger agent is initialized with a role and a set of states for that role. The challenger agent API is split and proveFraudOrForfeit. For now, the challenger agent is APIs are invoked manually by a player. The end goal is for the challenger agent to monitor an event stream. For example,
  • A challenger agent monitors rollup roots posted on chain. If the challenger agent detects an invalid root, the challenger agent initiates a challenge (currently implemented as the constructor).
  • The challenger agent monitors dispute game events and calls split or proveFraudOrForfeit in response to other player's actions.
• A DisputeGame is used for testing. In a production dispute, there is no DisputeGame
• The ChallengeManager now keeps track of the loser property and allows a player to forfeit.

[andrewgordstewart]

A challenger agent monitors rollup roots posted on chain. If the challenger agent detects an invalid root, the challenger agent initiates a challenge (currently implemented as the constructor).

This might be out of scope of a ChallengerAgent. A ChallengerAgent might be used only during a challenge, not to determine when to launch a challenge.

[kerzhner]

This might be out of scope of a ChallengerAgent. A ChallengerAgent might be used only during a challenge, not to determine when to launch a challenge.

I agree with that. Seems like there should be another component that monitors for when to launch a challenge.