RFR 8071600: Add a flat-mapping collector

[Paul Sandoz]

Hi,

http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping/webrev/

This patch adds a new flat mapping collector to Collectors. This can be useful if one needs to map 0 or more items into a downstream collector.

A CCC will be filed.

A following patch, which i plan to fold into the above patch, performs some renames on the collectors test to be consistent with the current naming: 

http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8071600-Collector-flatMapping-test-rename/webrev/

Thanks,
Paul.