Add MonadDi for sync and async logging functions

[k0001]

Di needs IO to work, we acknowledge that, but sometimes the Monad on which you are trying to use Di deliberately doesn't support MonadIO, likely so that it can provide a safe interface and prevent arbitrary IO for being called.

We propose changing functions like err :: MonadIO m => Di path msg -> msg -> m () to err :: MonadDi m => Di path msg -> msg -> m (), where:

class MonadDi (m :: * -> *) where
   liftLog :: Log -> m ()
   default liftLog :: MonadIO m => Log -> m ()
   liftLog = liftIO . runLog

newtype Log = UnsafeLog (IO ())  -- ^ Constructor not exported

runLog :: Log -> IO ()
runLog (Log m) = m 

The cost of doing this, of course, is that now we need to provide MonadDi instances for every monad transformer. We can do that for all of the ones in transformers.

Thanks @shlevy for the suggestion

[k0001]

liftLog is a terrible name.

[k0001]

I've decided against this.

The reason is that the only function where this matters is log, and if you do have some IO-based Monad Foo that doesn't implement MonadIO, then you can just do:

fooLog :: Monoid path => Di level path msg -> level -> msg -> Foo ()
fooLog = \di l m -> UnsafeFooWhatever (liftIO (Di.log di l m))

Doing this is cheaper that having to maintain a new MonadXxx typeclass which would need a new instance for every new monad.