RFR: JDK-8319123 : Implementation of JEP-461: Stream Gatherers (Preview) [v3]

[Viktor Klang]

> This Pull-Request implements [JEP-461](https://openjdk.org/jeps/461)

Viktor Klang has updated the pull request incrementally with two additional commits since the last revision:

 - Multiple improvements to Gatherer and Gatherers javadoc and restructuring of Gatherers.java to put public at the top of the file.
 - Augmenting Gatherer tests to include default implementation in Stream

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/6c05d682..74936cdd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=01-02

  Stats: 1036 lines in 6 files changed: 464 ins; 372 del; 200 mod
  Patch: https://git.openjdk.org/jdk/pull/16420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16420/head:pull/16420

PR: https://git.openjdk.org/jdk/pull/16420