RFR: 8319123: Implement JEP 461: Stream Gatherers (Preview) [v13]

[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:

  Javadoc clarifications for Gatherer type parameters,
  correcting descriptions of Integrator type parameters,
  and more consistently use "initializer function" instead
  of "supplier function".

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16420/files
  - new: https://git.openjdk.org/jdk/pull/16420/files/48b7e329..ec841740

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=12
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=11-12

  Stats: 13 lines in 1 file changed: 0 ins; 0 del; 13 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