RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v14]

[Viktor Klang]

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

Viktor Klang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 32 commits:

 - Lowering upper boundary of gatherer composition testing to 512
 - Javadoc clarifications for Gatherer type parameters,
   correcting descriptions of Integrator type parameters,
   and more consistently use "initializer function" instead
   of "supplier function".
 - Addressing trailing whitespace complaint
 - Adding @implSpec for Gatherer.andThen, plus adds tests to verify the default impls for Gatherer-related interfaces
 - Clarifying that Stream#gather is a stateful intermediate operation
 - Improvements after feedback
 - Unpacking the rethrown exceptions from Gatherers.mapConcurrent()
 - Addressing last review feedback
 - Apply suggestions from code review
   
   Integrates Paul Sandoz's feedback/changes
   
   Co-authored-by: Paul Sandoz <paul.d.sandoz at googlemail.com>
 - Minor improvements to Gatherer Javadoc
 - ... and 22 more: https://git.openjdk.org/jdk/compare/69413690...49c94c7f

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Changes: https://git.openjdk.org/jdk/pull/16420/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=13
  Stats: 4988 lines in 24 files changed: 4981 ins; 0 del; 7 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