RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v15]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Add test and comments about null keys

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23972/files
  - new: https://git.openjdk.org/jdk/pull/23972/files/5bdb5584..8c0ea1ab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23972&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23972&range=13-14

  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 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