RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v10]

[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:

  Clarifying that Stream#gather is a stateful intermediate operation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/ba84335e..2c7ac828

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=08-09

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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