UnliftIO-ify the API #84

Here is what unliftio has made of the async API:

https://hackage.haskell.org/package/unliftio-0.2.7.0/docs/UnliftIO-Async.html

The purpose of this abstraction is passing in transformer stacks on top of IO in the negative position, as in:

async :: MonadUnliftIO m => m a -> m (Async a)

What do you think of it? Assuming it is a good abstraction (I happen to think so but as far as I can tell the community is undecided), for organizational purposes, it would be great if async could pick up an unliftio-core dependency, then unliftio can stop mirroring the async API.

Thanks :)