[code-reflection] RFR: Externalizable operations [v3]

[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.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Renaming and relocating.

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Changes:
  - all: https://git.openjdk.org/babylon/pull/62/files
  - new: https://git.openjdk.org/babylon/pull/62/files/ab1a7412..82b490ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=62&range=02
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=62&range=01-02

  Stats: 454 lines in 9 files changed: 100 ins; 155 del; 199 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