rid::message and rid::reply

Explains the Rid message and reply attributes.

What is it?

A Rust enum can be annotated with the #[rid::reply] attribute.

This same enum can then be used when annotating another enum with the #[rid::message(Reply)] attribute.

This is how Messages and Replies are setup for a rid application.

How Rid sends Messages

TODO complete

How Rid receives Replies

TODO complete