RFR: Stripped down scope locals

[Andrew Haley]

Rework scope local support without inheritance.

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Commit messages:
 - Merge https://github.com/openjdk/loom fibers
 - Merge https://github.com/openjdk/loom fibers
 - Merge https://github.com/openjdk/loom fibers
 - Optimizations
 - Cleanup JavaDoc and tests.
 - Cleanups
 - Tweak scope-local cache randomization
 - Rename noninheritableScopeLocalBindings
 - Refactor ScopeLocal
 - Scope locals performance tests
 - ... and 20 more: https://git.openjdk.java.net/loom/compare/1cc08b47...6d5deb3f

Changes: https://git.openjdk.java.net/loom/pull/58/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=58&range=00
  Stats: 700 lines in 12 files changed: 27 ins; 568 del; 105 mod
  Patch: https://git.openjdk.java.net/loom/pull/58.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/58/head:pull/58

PR: https://git.openjdk.java.net/loom/pull/58