[code-reflection] RFR: Remove ExternalizableOp

[Paul Sandoz]

Remove `ExternalizableOp` and allow any `Op` to externalize its specific state. Remove `ExternalizableTypeElement` and allow any `TypeElement` to externalize its state.

Add a `Location` component to `ExternalizedOp`, instead of including in an externalized operations specific state (map of attributes).

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Commit messages:
 - Move Op.NULL_ATTRIBUTE_VALUE
 - Remove ExternalizableTypeElement
 - HAT updates.
 - Update triton example
 - Update ONNX examples
 - Cleanup
 - Start removing ExternalizableOp

Changes: https://git.openjdk.org/babylon/pull/464/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=464&range=00
  Stats: 1471 lines in 61 files changed: 347 ins; 769 del; 355 mod
  Patch: https://git.openjdk.org/babylon/pull/464.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/464/head:pull/464

PR: https://git.openjdk.org/babylon/pull/464