[code-reflection] RFR: Refactor type element

[Maurizio Cimadamore]

On Wed, 1 May 2024 21:42:41 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> It's possible to go further and define a subtype `ExternalizableCodeType` that supports externalization to `ExternalizedCodeType`, copying the pattern for operations.

Yeah. that would seem more consistent with what with did with `Op`s. Also I note there's an `externalize` method that throws UOE on non-externalizable types which seems to call for pattern match. (But that can some i a separate PR)

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PR Comment: https://git.openjdk.org/babylon/pull/70#issuecomment-2090132436