RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v12]

[Per Minborg]

> Implement JEP 502.
> 
> The PR passes tier1-tier3 tests.

Per Minborg has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 257 commits:

 - Remove link
 - Merge branch 'master' into implement-jep502
 - Improve exception checking
 - Remove empty instances
 - Remove snippet for orElseSet
 - Add partial equality test
 - Update src/java.base/share/classes/java/lang/StableValue.java
   
   Co-authored-by: Paul Sandoz <paul.d.sandoz at googlemail.com>
 - Revamp toString() methods
 - Fix comments on doc issues
 - Create separate reentry prevention method and add tests
 - ... and 247 more: https://git.openjdk.org/jdk/compare/59629f88...7fe2c363

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Changes: https://git.openjdk.org/jdk/pull/23972/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23972&range=11
  Stats: 4000 lines in 30 files changed: 3969 ins; 18 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/23972.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23972/head:pull/23972

PR: https://git.openjdk.org/jdk/pull/23972