[code-reflection] RFR: Remove ExternalizableOp [v2]

[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).

Paul Sandoz has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/code-reflection' into rm-ex-op
 - Move Op.NULL_ATTRIBUTE_VALUE
 - Remove ExternalizableTypeElement
 - HAT updates.
 - Update triton example
 - Update ONNX examples
 - Cleanup
 - Start removing ExternalizableOp

-------------

Changes:
  - all: https://git.openjdk.org/babylon/pull/464/files
  - new: https://git.openjdk.org/babylon/pull/464/files/228f7df7..3ee3ecff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=464&range=01
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=464&range=00-01

  Stats: 701 lines in 8 files changed: 609 ins; 84 del; 8 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