[code-reflection] RFR: Structured type definition

[Paul Sandoz]

Replace the currently implementation of `TypeDefinition` with a more general tree structure, which is the boundary between the runtime and serialized form.

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Commit messages:
 - Structured type definition.

Changes: https://git.openjdk.org/babylon/pull/23/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=23&range=00
  Stats: 436 lines in 17 files changed: 110 ins; 233 del; 93 mod
  Patch: https://git.openjdk.org/babylon/pull/23.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/23/head:pull/23

PR: https://git.openjdk.org/babylon/pull/23