Possible Monad instance for Data.Stream.Future

I have an idea on the type `Data.Stream.Future` provided by this package.
I'm not sure if it's good or not, but let me explain.

`Data.Stream.Future.Future` can have a `Monad` instance below. Like the instance of `Stream`, this Monad joins `Future (Future a))` by taking diagonal.

```haskell
-- | 'index' but returns the last value for indices larger than
-- the last index.
--
-- It's 'predict' because it guesses the stream will repeat
-- the same value after the last index.
--
-- > predict 0 = extract
predict :: Int -> Future a -> a
predict n aas
| n < 0 = error "predict: negative index"
| otherwise = case aas of
Last a -> a
a :< as | n == 0 -> a
| otherwise -> predict (n - 1) as

instance Monad Future where
aas >>= k = join' (fmap k aas)
where
join' = go 0
go !i (bs :< bss) = predict i bs :< go (succ i) bss
go !i (Last bs) = predict i $ duplicate bs
```

This behavior is incompatible with the current `Applicative Future` instance.
The current `Applicative` zips two `Future` streams by cutting the longer stream to the length of the shorter stream.
The compatible `Applicative` zips two `Future` streams by extending the shorter stream to the length of the longer stream, repeating the last value.