[code-reflection] RFR: Debug appropriate code element toString

[Paul Sandoz]

The `toString` of code elements (op, body, block) can be a little less verbose and better for debugging, especially so for blocks where we can include the index.

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Commit messages:
 - Debug appropriate code element toString

Changes: https://git.openjdk.org/babylon/pull/436/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=436&range=00
  Stats: 20 lines in 3 files changed: 20 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/babylon/pull/436.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/436/head:pull/436

PR: https://git.openjdk.org/babylon/pull/436