Use codable models in API

This MR contains !9 (merged) and !11 (merged) (make sure to merge them before reviewing this MR)

• Update IdentityMaker to return Identity model.
• Update ConnectionMaker to accept Identity model.
• Use Message model in MessageListener.
• Use RestlikeMessage model in RestlikeRequestSender.
• Use MessageSendReport in MessageDeliveryWaiter.
• Use [Int] for rounds in MessageDeliveryWaiter.Result.delivered.
• Use Identity model in ContactFromIdentityProvider.
• Use Fact model in ContactFactsProvider.
• Use Fact model in ContactFactsSetter.