RFR: Robust take 3

[Andrew Haley]

Latest draft of robust extent locals.

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Commit messages:
 - Merge https://github.com/openjdk/loom into robust-take-3
 - More
 - More
 - More
 - Merge https://github.com/openjdk/loom into robust-take-3
 - Reorg
 - Comments
 - Better
 - Better
 - Better
 - ... and 40 more: https://git.openjdk.org/loom/compare/3e1aa292...aefe6704

Changes: https://git.openjdk.org/loom/pull/190/files
 Webrev: https://webrevs.openjdk.org/?repo=loom&pr=190&range=00
  Stats: 239 lines in 16 files changed: 191 ins; 22 del; 26 mod
  Patch: https://git.openjdk.org/loom/pull/190.diff
  Fetch: git fetch https://git.openjdk.org/loom pull/190/head:pull/190

PR: https://git.openjdk.org/loom/pull/190