Use one executable for both worker and server.

[nominolo]

I'm not sure this is such a good idea because its only advantage is reduced linking time when building. Of course, as long as we're below version 1.0 this might be a worthwhile "feature'.

[nominolo]

Currently the Scion server does not compile with GHC 6.12.* because atto-lisp does not compile with that version of GHC (due to some type-level hackery). In the future we want to be able run workers compiled with different versions of GHC (but with the same version of Scion and binary). Since the workers use a binary protocol to communicate with the server it doesn't matter which version of GHC they were compiled with.