[code-reflection] RFR: Create CoreType

[Paul Sandoz]

Create a common type, `CoreType`, and consolidate factory methods in that class.

Create an `ExternalizableTypeElement` that all type elements wishing to explicitly support externalization should implement, following the same pattern as `ExternalizableOp`.

The method `TypeElement::externalize` will be revisited in a subsequent PR, likely removed.

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A subsequent PR will consolidate packages `parser` and `writer` into an `extern` package. The classes directly under the `dialect` will be moved to be directly under the `extern` package. Thereby we consolidate all externalization/reconstitution functionality under the same package.

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Commit messages:
 - Merge remote-tracking branch 'upstream/code-reflection' into core-type
 - Move ExternalizableTypeElement and TypeElementFactory.
 - Updates to HAT
 - Merge remote-tracking branch 'upstream/code-reflection' into core-type
 - Cleanups
 - Cleanups.
 - Cleanups.
 - Cleanups.
 - Cleanups.
 - Create CoreType.

Changes: https://git.openjdk.org/babylon/pull/454/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=454&range=00
  Stats: 870 lines in 64 files changed: 333 ins; 279 del; 258 mod
  Patch: https://git.openjdk.org/babylon/pull/454.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/454/head:pull/454

PR: https://git.openjdk.org/babylon/pull/454