[DPP-52] Unified API for making AsyncTask

[qnikst]

[Imported from JIRA. Reported by Tim Watson [Administrator] @hyperthunk) as DPP-52 on 2013-01-11 14:22:47] At the moment we have asyncDo but that's pretty ugly, leaving a default local spawn looking like this:

h <- async $ asyncDo $ do ..... 

Urgh. I suspect type synonym families might help here.