Pass appropriate java.util.stream.Collector.Characteristics to Collector API

[lukaseder]

java.util.stream.Stream can implement some optimisations if custom Collectors pass hints to the collect() API via java.util.stream.Collector.Characteristics. We should also pass these characteristics.

[amaembo]

As you mostly ignore parallel streams, the only useful characteristic in your case is IDENTITY_FINISH which is automatically injected if you are using Collector.of without providing the finisher.

[lukaseder]

Thanks for the feedback, @amaembo. Within jOOλ, that's certainly true, but people might use the collectors exposed in Agg also for parallel JDK streams, or for StreamEx...