RFR: JDK-8319123 : Implementation of JEP-461: Stream Gatherers (Preview)

[Viktor Klang]

This is a Draft PR for [JEP-461](https://openjdk.org/jeps/461)

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Commit messages:
 - Updating Javadoc for Gatherer and Gatherers based
 - Addressing feedback on Gatherers
 - Removing Gatherers.peek() and Gatherers.peekOrdered()
 - Updates after review comments
 - Removing trailing whitespace
 - Formatting and Javadoc improvements to JEP-461
 - Additional improvements to Gatherer Javadoc
 - Minor adjustments to Javadoc for Gatherers
 - Updating Gatherers preview name and JEP number
 - Adding --enable-preview to Gatherer benchmarks
 - ... and 7 more: https://git.openjdk.org/jdk/compare/36de19d4...2206e04d

Changes: https://git.openjdk.org/jdk/pull/16420/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16420&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319123
  Stats: 4803 lines in 24 files changed: 4796 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