p9: refactor message types into their own package

[DeedleFake]

The primary package of the module is currently full of implementation details that really probably don't need to be in the same package as everything else, such as the various T* and R* types. They clog up the documentation, making it more awkward to just browse through the list of important stuff. They should probably be refactored into a new package, maybe called something like msg.