RFR: 8341903: Implementation of Scoped Values (Fourth Preview) [v4]

[Andrew Haley]

> The fourth preview of scoped values.

Andrew Haley has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge branch 'clean' into JDK-8341903
 - Scoped values
 - Scoped values
 - Fix javadoc
 - Scoped Values API changes
 - Scoped Values API changes
 - Scoped Values API changes

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21456/files
  - new: https://git.openjdk.org/jdk/pull/21456/files/90a344a3..052553ea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21456&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21456&range=02-03

  Stats: 7874 lines in 151 files changed: 7141 ins; 234 del; 499 mod
  Patch: https://git.openjdk.org/jdk/pull/21456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21456/head:pull/21456

PR: https://git.openjdk.org/jdk/pull/21456