What

Replace the Double_preattestation_evidence and Double_attestation_evidence operations with a new Double_consensus_operation_evidence operation. Also add a denounced slot field.

For the evidence to be valid, the denounced operations must both be preattestations (each one may be aggregated or not) or both be attestations. Moreover, both must involve the denounced slot, that is, be either a standalone operation for this slot or an aggregate whose committee includes this slot.