Derive Macro MultiSendMessage

#[derive(MultiSendMessage)]
{
    // Attributes available to this derive:
    #[multi_send_message_derive]
    #[multi_send_input_derive]
}

Derives two enums, whose names are based on this struct by appending Message and Input. Each enum has a case for each Sender or AsyncSender in this struct. The Message enum contains the raw message being sent, which is X for Sender<X> and MessageWithCallback<X, Y> for AsyncSender<X, Y>. The Input enum contains the same for Sender but only the input, X for AsyncSender<X, Y>.

Additionally, this struct can then be used to .send using the derived Message enum. This is useful for packaging a multi-sender as a singular Sender that can then be embedded into another multi-sender. The Input enum is useful for capturing messages for testing purposes.