[code-reflection] RFR: Externalizable operations

[Paul Sandoz]

Rename `OpWithDefinition` to `ExternalizableOp` and specify as an operation whose contents may be externalized.
Rename `OpDefintion` to `ExternalOpContents`, an instance of which represents the external contents of an operation.
An externaliziable operation may be constructed from its external contents, for example by using an operation factory. 

The method `Op::attributes` is removed, and the method is defined on `ExternalizableOp`. The attribute map returned by the method is the external representation of specific contents of an operation.

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Commit messages:
 - Merge branch 'code-reflection' into extern-op
 - Update Triton example.
 - Updates
 - Externalizable operations.

Changes: https://git.openjdk.org/babylon/pull/62/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=62&range=00
  Stats: 688 lines in 18 files changed: 227 ins; 216 del; 245 mod
  Patch: https://git.openjdk.org/babylon/pull/62.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/62/head:pull/62

PR: https://git.openjdk.org/babylon/pull/62