Integrated: Scope locals next

[Andrew Haley]

Partial rewrite of Scope Locals support. Faster, main class renamed, lots of good stuff.

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Commit messages:
 - Rename java.lang.Scoped to ScopeLocal
 - Fix callWithSnapshot tests
 - Fix CompletableFuture
 - Add subclasses of CompletableFuture to Scoped snapshots
 - Merge https://github.com/openjdk/loom into scope-locals-next
 - Add subclasses of CompletableFuture to Scoped snapshots
 - Simplify, restructure.
 - Cleanup
 - Cleanup
 - Merge https://github.com/openjdk/loom into fibers
 - ... and 10 more: https://git.openjdk.java.net/loom/compare/c3b70c83...ab629de4

Changes: https://git.openjdk.java.net/loom/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=34&range=00
  Stats: 2041 lines in 24 files changed: 998 ins; 971 del; 72 mod
  Patch: https://git.openjdk.java.net/loom/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/34/head:pull/34

PR: https://git.openjdk.java.net/loom/pull/34