Remove the honest defender assumption

[norswap]

The current challenge game assumes that the defender acts honestly.

Things that are needed, at minimum, we need to introduce a challenge period after which the last player to answer in the binary search wins by default if the binary search.

I would also like to take care of the issue that arises when the challenger and the defender disagree on the last step of the challenge game. In this case, the defender won't have supplied a state hash for the post state of that transition, and thus it can not prove the challenger wrong (but wins by default if the challenger doesn't prove he's right). I'd like to at least get ride of the ChallengerLosesByDefault event (as the previous paragraph implies a robust system for determining who loses by default if the challenge period expires without a clear winner). It might also make sense to allow the defender to provide the post state when proving the last step (in which case we must explicitly verify that the assert post-state is different than the one asserted by the challenger).

[protolambda]

Challenge-response game is out of scope for Cannon, and split into a sepearate Dispute Game project. Closing this.