[lworld] RFR: 8301720: Add the PreviewFeature "VALUE_OBJECTS" and annotate new APIs.

[Roger Riggs]

Add the PreviewFeature for value objects and annotate new APIs.
(Any restrictions on use of the APIs will come later.)

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

Commit messages:
 - 8301720: Add the PreviewFeature "VALUE_OBJECTS" and annotate new APIs.

Changes: https://git.openjdk.org/valhalla/pull/819/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=819&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301720
  Stats: 20 lines in 4 files changed: 18 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/819.diff
  Fetch: git fetch https://git.openjdk.org/valhalla pull/819/head:pull/819

PR: https://git.openjdk.org/valhalla/pull/819