RFR : 8010953: Add primitive summary statistics utils

[Mike Duigou]

Hello all;

Another integration review in the JSR-335 libraries series. These three classes provide a utility for conveniently finding count, sum, min,  max and average of ints, longs or doubles. They can be used with existing code but will most likely be used with the Collectors utilities or directly with primitive or boxed streams.

http://cr.openjdk.java.net/~mduigou/JDK-8010953/1/webrev/

(this is an updated version of the webrev sent to core-libs-dev).

Mike