Add Control.Monad.Free.toFreeT

This pull request introduces:

```haskell
Control.Monad.Free.toFreeT 
  :: (Functor f, Monad m)
  => Control.Monad.Free.Free f a
  -> Control.Monad.Trans.Free.FreeT f m a 
```
I've found this small utility useful when you implements a free monad interpreter for `Control.Monad.Trans.Free.FreeT`, yet you expose some parts of your API using just `Control.Monad.Free.Free` for simplicity.

I put it in `Control.Monad.Free` because it's easier to find a name for this function there, as `Control.Monad.Trans.Free` already exports something named `Free` which is different from the `Free` we care about in this function.