RFR: 8319123: Implementation of JEP-461: Stream Gatherers (Preview) [v8]

[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:

  Unpacking the rethrown exceptions from Gatherers.mapConcurrent()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/62082a6a..bc8f2364

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=06-07

  Stats: 23 lines in 2 files changed: 10 ins; 2 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