RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v9]

[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:

  Improvements after feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/bc8f2364..ba84335e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=07-08

  Stats: 57 lines in 2 files changed: 17 ins; 18 del; 22 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