Collectors inventory

[Brian Goetz]

Breaking this into multiple messages.  

> 3. Move toStatistics directly to IntStream, LongStream, DoubleStream instead of using stream.collect(toStatistics())

No objection to adding these to IntStream and friends (one-liner), but I think it also has to stay in Collectors, so that you can do things like gather statistics on properties of object streams, such as "sales volume statistics by salesman" queries.