Add evidence curation support

The scheme below is not backwards compatible. We can build a system that is backwards compatible with arbitrable contracts already deployed.

~# Summary~

~We can use the Generalized TCR contract to build an evidence section visually similar to reddit's comment section. Additionally, evidence submission can require a deposit such that anyone can challenge it.~

~To do this, we add a new field address evidenceTCR to the request struct. We update challengeRequest() to also:~ ~1- Deploy a new GTCR contract;~ ~2- Save its address in request.evidenceTCR;~ ~3- Call evidenceTCR.addItem with _evidence~

~Additionally, we update the constructor() to accept parameters that will be passed to the constructor of the evidence TCR.~

~### Example~ ~Consider a Token TCR at a given point in time:~ ~challengeBaseDeposit: 1 wei~ ~evidenceChallengeBaseDeposit: 1 wei~

~A user challenges a request, which which deploys the evidence TCR. For the constructor parameters, the evidence arguments are passed such that child evidence disputes will have the same context as the parent:~

request.evidenceTCR = new GeneralizedTCR(
    ...,
  evidenceChallengeBaseDeposit, // This will be stored in challengeBaseDeposit
  evidenceChallengeBaseDeposit, // This will be stored in evidenceChallengeBaseDeposit
   ... 
);

~This way, the price to challenge a token submission is different than the deposit to challenge the evidence of a token submission dispute. However, the deposit to challenge evidence related to the token dispute will be the same no matter how deep in the tree it is.~

~## Note on EIP-170~ ~The queryItems function was added to the GeneralizedTCRView contract, which means we can remove it from the GeneralizedTCR contract if the above modifications cause the bytecode to go over 24.5kb in size.~

https://forum.kleros.io/t/feature-request-ability-to-mark-evidence-as-fake-news/247/3

kleros / tcr

Add evidence curation support #4