Message should io.WriterTo and io.ReaderFrom

[lthibault]

As per #257 this effort would avoid the need to instantiate an encoder/decoder for single messages.

@zenhack My interpretation of your comment is that this is pure syntactic sugar, and so I would expect these methods to instantiate and call an encoder/decoder under the hood. Have I correctly understood your intent?

[zenhack]

Yes, that's what I had in mind.