RFR: Initial scoped locals implementation

[Andrew Haley]

Here it is. Still no Structured Concurrency support, though.

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Commits:
 - fa83f1ae: Initial scoped locals implementation

Changes: https://git.openjdk.java.net/loom/pull/1/files
 Webrev: https://webrevs.openjdk.java.net/loom/1/webrev.00
  Stats: 907 lines in 23 files changed: 905 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/loom/pull/1.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/1/head:pull/1

PR: https://git.openjdk.java.net/loom/pull/1