RFR 8009736: Comparator API cleanup

[Henry Jen]

Hi,

Please review http://cr.openjdk.java.net/~henryjen/ccc/8009736.2/webrev/

Highlight of changes,

- Comparators class is now only package-private implementations. The
public static methods have been move to other arguably more appropriate
places, mostly in Comparator.

- Comparator.reverseOrder() is renamed to Comparator.reversed()

- nullsFirst(Comparator) and nullsLast(Comparator) are introduced to
wrap up a comparator to be null-friendly.

To see the API changes, found the specdiff at
http://cr.openjdk.java.net/~henryjen/ccc/8009736.2/specdiff/overview-summary.html

Cheers,
Henry