Codec generator?

I think it would be worth to create JSON-Codec and Buffer-Codec for the generated messages, as it would reduce some of the boilerplate in the client's code. Maybe as an option in the generated output? A simple example is the following, the extension could be passed in too:

```
class _Message__BufferEncoder extends Converter> {
List convert(Message input) =>
input.writeToBuffer();
}

class _Message__BufferDecoder extends Converter, Message> {
Message convert(List input) =>
new Message.fromBuffer(input);
}

class Message__BufferCodec extends Codec> {
Converter _decoder = new _Message__BufferDecoder();
Converter _encoder = new _Message__BufferEncoder();
Converter, Message> get decoder => _decoder;
Converter> get encoder => _encoder;
}
```