Collector / Collectors

[Brian Goetz]

I think I may (finally!) have a final API candidate for Collector / 
Collectors.  Updated docs here:

   http://cr.openjdk.java.net/~briangoetz/doctmp/doc/

Salient recent changes:
  - Incorporation of finishing function into Collector
  - Renamings in Collector API
  - Morph toStringBuilder, toStringJoiner into joining()
  - Add prefix/suffix version of joining()
  - Addition of summingBy{Int,Long,Double}
  - Addition of averagingBy{Int,Long,Double}
  - Rename toXxxSummaryStatistics to summarizingXxx
  - no-see reducing, and minBy/maxBy collectors return Optional
  - elimination of canned merger functions from API

Please review the API and spec.  I have set up a SurveyMonkey with the 
usual password to collect review comments at:

   https://www.surveymonkey.com/s/3TTBJ7K