Make Transport.Empty externally available in the Haskell Thrift Library

The Haskell Thrift library contains an "Empty" transport. It looks to me that this is the easiest way to use Thrift in Haskell for serialization-only purposes, but it is currently not exposed to users of the library. It is not used internally either, so I think this is just an oversight, and I would suggest to add it to the exported modules.