Gatherer: spliterator characteristics are not propagated

[Remi Forax]

While doing some benchmarking of the Gatherer API, i've found that the characteristics of the spliterator was not propagated by the method Stream.gather() making the stream slower than it should.

As an example, there is no way when reimplementing map() using a Gatherer to say that this intermediate operation keep the size, which is important if the terminal operation is toList() because if the size is known, toList() will presize the List and avoid the creation of an intermediary ArrayList.

See https://github.com/forax/we_are_all_to_gather/blob/master/src/main/java/com/gihtub/forax/wearealltogather/bench/MapGathererBenchmark.java

I think that adding a way to propagate the spliterator characteristics through a Gatherer would greatly improve the performance of commons streams (at least all the ones that end with a call to toList).

I have some idea of how to do that, but I prefer first to hear if i've overlook something and if improving the performance is something worth changing the API.

regards,
Rémi