[code-reflection] Integrated: Simplify CodeElement.elements

[Paul Sandoz]

Simplify CodeElement.elements.

Update TestExpressionGraphs to use streams only and be in sync with https://openjdk.org/projects/babylon/articles/code-models

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Commit messages:
 - Add back comment.
 - Simplify CodeElement.elements

Changes: https://git.openjdk.org/babylon/pull/728/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=728&range=00
  Stats: 37 lines in 2 files changed: 4 ins; 26 del; 7 mod
  Patch: https://git.openjdk.org/babylon/pull/728.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/728/head:pull/728

PR: https://git.openjdk.org/babylon/pull/728