Refactor Protocol Frame Enum to Struct with channelID property

[funcmike]

From

    public typealias ChannelID = UInt16

    case method(ChannelID, Method)
    case header(ChannelID, Header)
    case body(ChannelID, body: ByteBuffer)
    case heartbeat(ChannelID)
}

To

public struct Frame: PayloadDecodable, PayloadEncodable {
    var channelID: ChannelID

    var payload: Payload

    enum Payload {
        case method(Method)
        case header(Header)
        case body(ByteBuffer)
        case heartbeat
    }
}