[code-reflection] RFR: Some HAT cleanups

[Paul Sandoz]

- Fix hatrun.sh to use the correct paths (i think they are corrected, and verified for my local build)
- Enhance  CMakeLists.txt to check if BEEHIVE env is set, so as not to hardcode the location
- Use more idiomatic code model APIs (some added based on HAT feedback)

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Commit messages:
 - Some HAT cleanups

Changes: https://git.openjdk.org/babylon/pull/141/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=141&range=00
  Stats: 79 lines in 10 files changed: 17 ins; 18 del; 44 mod
  Patch: https://git.openjdk.org/babylon/pull/141.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/141/head:pull/141

PR: https://git.openjdk.org/babylon/pull/141