[code-reflection] RFR: Obtain parent code element from code element

[Paul Sandoz]

`CodeElement` allows access to its children but not its parent. We can add a method to support that.

Subclasses `Op`, `Body`, and `Block` have duplicate methods for accessing their parent and children (the subclasses have covariant overrides of the `CodeElement` methods) e.g., `CodeElement::parent` and Op::parentBlock,  `CodeElement::children` and `Op::bodies`. We may not require the subclass specific methods, but i am leaving there for now.

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Commit messages:
 - Obtain parent code element from code element

Changes: https://git.openjdk.org/babylon/pull/35/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=35&range=00
  Stats: 146 lines in 4 files changed: 92 ins; 50 del; 4 mod
  Patch: https://git.openjdk.org/babylon/pull/35.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/35/head:pull/35

PR: https://git.openjdk.org/babylon/pull/35