RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v11]

[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:

  Adding @implSpec for Gatherer.andThen, plus adds tests to verify the default impls for Gatherer-related interfaces

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/2c7ac828..080bd484

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=09-10

  Stats: 34 lines in 2 files changed: 33 ins; 0 del; 1 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