RFR: JDK-8011917 (java.util.stream.Collectors)

Single new source file at: http://hg.openjdk.java.net/lambda/lambda/jdk/file/76ac19e61df1/src/share/cla... Doc at: http://cr.openjdk.java.net/~briangoetz/JDK-8008682/api/java/util/stream/Coll... for JDK-8011917 (Collectors class in java.util.stream). (No webrev as there's just one new file.)