SCADS specific code in comm should be moved into scalaengine

[rxin]

My understanding is that the comm folder provides the infrastructure for messaging support.

Most stuff in Messages.scala are really only relevant to SCADS-engine. They should probably be moved into engine.

[marmbrus]

while i agree with this idea in principle, its going to require a fair amount of refactoring. Right now the message handler is hard coded to accept a certain set of messages. This is why these messages are defined in comm and not scalaengine.

[stephentu]

another reason that messages are defined in comm is due to issues w/ the typer, which is a limitation of the scala compiler plugin framework (messages defined in the same compliation pass are typechecked before the auto-generated methods are added, but we need the typer to run before we can auto-generate methods).