sdk%feat: switch from {En,De}codable to {En,De}code, replace free-function API with Recipient enum, adopt bitcoin_p2p_messages crate types into p2p_core

sdk%feat: switch from {En,De}codable to {En,De}code, replace free-function API with Recipient enum, adopt bitcoin_p2p_messages crate types into p2p_core #126