Support for RPC across ZeroMQ

[brotchie]

Preliminary Haskell support for RPC across ZeroMQ Rep-Req sockets.

This pull request aims to stimulate discussion to find the best approach for abstracting the transport layer from the Haskell msgpack RPC implementation. I do not intend to have this merged as-is into master.

My currently implementation isn't elegant; largely a blatant copy-paste with network conduits replaced with zeromq calls. Perhaps the optimal approach is to write a zeromq conduit such that the RPC server and client is indifferent to the underlying conduit source?

[tanakh]

Sorry for late reply. I try to merge it and it works fine. I rewrite server independently to use network-conduit, and allow MonadTrans. And network-conduit's interface (Source -> Sink -> m ()) is seems to be likely to use transport layer's interface. It is simple and flexible. I adopt your patch to this interface. What do you think about it?