RFR: JDK-8319123 : Implementation of JEP-461: Stream Gatherers (Preview) [v6]

[Viktor Klang]

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

Viktor Klang has updated the pull request incrementally with one additional commit since the last revision:

  Apply suggestions from code review
  
  Integrates Paul Sandoz's feedback/changes
  
  Co-authored-by: Paul Sandoz <paul.d.sandoz at googlemail.com>

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/b1540ca5..6a6c1fb1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=04-05

  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 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