Offer stricter versions of AccumT

[original issue 89 by @treeowl]

AccumT is extremely lazy, much like lazy StateT. I suspect that for most purposes, what users actually want is a version that's strict in the accumulator value. That would correspond to instances (at least approximately) like the following:

instance (Monoid w, Functor m, Monad m) => Applicative (AccumT w m) where

    pure a  = AccumT $ \ !_ -> return (a, mempty)

    {-# INLINE pure #-}

    mf <*> mv = AccumT $ \ !w -> do

      (f, !w&#39;)  <- runAccumT mf w

      (v, !w&#39;&#39;) <- runAccumT mv (w `mappend` w&#39;)

      return (f v, w&#39; `mappend` w&#39;&#39;)

    {-# INLINE (<*>) #-}


instance (Monoid w, Functor m, Monad m) => Monad (AccumT w m) where

    m >>= k  = AccumT $ \ !w -> do

        (a, !w&#39;)  <- runAccumT m w

        (b, !w&#39;&#39;) <- runAccumT (k a) (w `mappend` w&#39;)

        return (b, w&#39; `mappend` w&#39;&#39;)

    {-# INLINE (>>=) #-}

I think it's also plausible that some users will want a version that's strict in the result pairs, but not in the accumulators. Would you be open to offering the totally strict version, and perhaps the pair-strict version?