RFR: 8306647: Implementation of Structured Concurrency (Preview) [v3]

[Alan Bateman]

> This is the implementation of:
> 
> - JEP 453: Structured Concurrency (Preview)
> - JEP 446: Scoped Values (Preview)
> 
> For the most part, this is just moving code and tests.  StructuredTaskScope moves to j.u.concurrent as a preview API, ScopedValue moves to j.lang as a preview API, and module jdk.incubator.concurrent has been removed. The significant API changes since incubator are:
> 
> - StructuredTaskScope.fork returns Subtask instead of Future (JEP 453 has a section on this)
> - ScopedValue.where methods are replaced with runWhere, callWhere and getWhere

Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:

  Sync with loom repo, re-work ScopedValue class description

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13932/files
  - new: https://git.openjdk.org/jdk/pull/13932/files/e92ba524..fce838e8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13932&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13932&range=01-02

  Stats: 176 lines in 5 files changed: 78 ins; 16 del; 82 mod
  Patch: https://git.openjdk.org/jdk/pull/13932.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13932/head:pull/13932

PR: https://git.openjdk.org/jdk/pull/13932