Add a fork/major version for GHC >= 8.6

As you've already seen, GHC 8.6 wins

```haskell
instance (MFunctor f, MFunctor g, forall m. Monad m => Monad (g m))
=> MFunctor (ComposeT f g)
```

But it has more to offer. We can change the `MonadTrans` instance to the (effectively) less-constrained, and sometimes more efficient,

```haskell
instance (MonadTrans f, MonadTrans g, forall m. Monad m => Monad (g m)) => MonadTrans (ComposeT f g)
where
lift = ComposeT . lift . lift
```

and then even change the type of `hoist` to

```haskell
hoist :: (Monad m, Monad n) => (forall a . m a -> n a) -> t m b -> t n b
```

which allows some important instances including `streamly` streams and Church-style free monad transformers.

Basically, aside from the fact that `QuantifiedConstraints` is a pain and a half for instance resolution, everything gets much nicer.