[code-reflection] Integrated: Simplifying InvokePtr example

[Gary Frost]

Some mildly cosmetic changes to InvokePtr expermental code, in prep for Schema and HatPtr work

Created a common interface type Builder, Builder.StructChild an Builder.UnionChild all extend MappableIface

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Commit messages:
 - Simplifying InvokePtr example

Changes: https://git.openjdk.org/babylon/pull/166/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=166&range=00
  Stats: 567 lines in 7 files changed: 102 ins; 311 del; 154 mod
  Patch: https://git.openjdk.org/babylon/pull/166.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/166/head:pull/166

PR: https://git.openjdk.org/babylon/pull/166