RFR: Latest scope locals changes.

[Andrew Haley]

In particular, support for Structured Concurrency, but also some JMH tests.

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Commit messages:
 - New test
 - New test
 - New test
 - Back out HotSpot support for inheritance from Unified-Carrier branch
 - Enable preview features in microbenchmarks.
 - Add EmptySnapshot
 - Add special-case scope local inheritance for StructuredThreadExecutors
 - Merge HotSpot support for inheritance from Unified-Carrier branch
 - Merge branch 'fibers' of https://github.com/openjdk/loom into fibers
 - Merge https://github.com/openjdk/loom into fibers
 - ... and 15 more: https://git.openjdk.java.net/loom/compare/535a3a73...ecf2160f

Changes: https://git.openjdk.java.net/loom/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=46&range=00
  Stats: 492 lines in 7 files changed: 459 ins; 25 del; 8 mod
  Patch: https://git.openjdk.java.net/loom/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/46/head:pull/46

PR: https://git.openjdk.java.net/loom/pull/46