Support list fusion to various toList operation

Rise three feature requests here ; )

+ Support list fusion when unpacking arrays, this's important for speeding up common use case, here's [some snippet](https://github.com/haskell-stdio/stdio/blob/master/Data/Vector.hs#L566) that maybe useful. And we can get fast fold for free.

+ Support reversed order toListR/unpackR, this can only be done at unpacking cheaply, and fast lazy foldl rely on this.

+ Add docs suggesting using `fromList . XXX . toList` idiom to take the advantage of fusion, i.e. reduce memory usage by eliminate intermediate representation (often a time vs space choice here).