Async Channel support

[hyperthunk]

Once we've cracked issue #8 (and have passing tests) then we should really extend the Control.Distributed.Platform.Async module to support Channels too. For this extension, I think it actually does make sense to make the async task definition a function whose type is aware of the reply channel

-- NB: AsyncTask is a replacement for SpawnAsync

-- | A task to be performed asynchronously. This can either take the
-- form of an action that runs over some type @a@ in the @Process@ monad,
-- or a tuple that adds the node on which the asynchronous task should be
-- spawned - in the @Process a@ case the task is spawned on the local node
type AsyncTask a = Process a | (NodeId, Process a)

-- | A task to be performed asynchronously using typed channels
type AsyncChanTask a = SendPort a -> Process () | (NodeId, SendPort a -> Process ())

``

It is **possible** that the implementation of `Async` (in issue #8) could change to use channels internally anyway, at which point this might become a moot point.

[rodlogic]

fyi:See this pull request by Simon Marlow:

haskell-distributed/distributed-process@847abf4

It seems to address the issue of efficiently receiving channel and process messages.

[hyperthunk]

The async-refactoring branch now contains two async modules, one for STM based interactions and another one that uses channels. Resolving as a duplicate of issue #38.