Allow seeding k3 randomness and return index of invalid label

[poszu]

Each verifier must verify a different subset of k3 indices to support distributed verification. To achieve that, the randomness seed used to select k3 indices must include a unique value for the verifying node (public key, p2p peer ID, etc.).

To create a malfeasance proof, we need to know which index in the proof is invalid. The result from the verification method should include this information.