Implement merkle proofs

[lalexgap]

Constructs and validates merkle proofs for the consensus and dispute node.

split and detectFraud now take in a new WitnessProof type

type Proof = MerkleToolsProof<string>[];
export type WitnessProof = {
  witness: Hash;
  proof: Proof;
};

The proof is a merkle proof generated by merkle-tools which looks like this:

 proof = [
   { right: '09096dbc49b7909917e13b795ebf289ace50b870440f10424af8845fb7761ea5' },
   { right: 'ed2456914e48c1e17b7bd922177291ef8b7f553edf1b1f66b6fc1a076524b22f' },
   { left: 'eac53dde9661daf47a428efea28c81a021c06d64f98eeabbdcff442d992153a8' }
 ]

The ChallengeManager has been updated to:

• keep track of the current merkle root.
• Validate merkle proofs using this.root and the provided WitnessProof