Remove message module

[andrejewski]

This removes the raj/message module.

Presenting Raj to a group it seemed that the focus fell onto the syntax of pattern matching and how state changes are defined. Those concerns are not really relevant to Raj as there is no assumption made how you model your state changes, just that there are state changes.

While I prefer raj/message style messages and pattern matching, I think it is going to be problematic for people trying to build around Raj if raj/message is taken to be the approved way of doing things, especially since the API would have been very minimal forever.

The module had only ever been included for convenience and had no strong ties to any other module. Before it becomes too pivotal, we should remove it.

I have created tagmeme which takes raj/message and improves upon my original API and is even more strict about the style I prefer. This means someone can create a library that better suits their use case as well. It is no longer a Raj concern.