Use Message trait for serio channels

[n-lebel]

This PR changes the signatures of the serio::{Sink, Stream} traits slightly so that the types sent have to be Serialize + Deserialize. This is so we can add a Scribed decorator on top of serio channels which appends all messages to a transcript; that was difficult to do in the Stream case because there was no straightforward way to access the bytes of the message. I don't expect this to affect anything downstream since sent types are meant to be Serialize + Deserialize anyway, but open to other ideas.

[th4s]

Hmm, usually it makes sense to separate Serialize and Deserialize. Maybe you can introduce some sort of wrapper trait in your library like e.g. trait SerializeStream: Stream { ... } which attaches your required trait bounds in the trait methods which then just delegate to the Stream methods?

[sinui0]

Perhaps a better place to plug in would be through the Deserializer trait which does have access to the serialized data. I would like to avoid adding the Serialize bound to the receiving side, and vice versa.