[code-reflection] RFR: Clean up support for boxing

[Maurizio Cimadamore]

This PR does a couple of things:
* it sharpens the types of the various `JavaType` constants to the correct sealed sub-interface
* it consolidates support for boxing/unboxing. More specifically there's now a pair of functions:


ClassType::unbox() -> Optional<PrimitiveType>


and


PrimitiveType::box() -> Optional<ClassType>


Note that both functions are partial: there are classes that are not wrappers (e.g. String) and there are primitive types that cannot be boxed (e.g. `void`).

I'm on the fence whether to use `Optional` for this, or just return null. Suggestions welcome.

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Commit messages:
 - Minor API tweaks
 - Merge branch 'code-reflection' into cleanup_type_factories
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/67/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=67&range=00
  Stats: 100 lines in 5 files changed: 42 ins; 22 del; 36 mod
  Patch: https://git.openjdk.org/babylon/pull/67.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/67/head:pull/67

PR: https://git.openjdk.org/babylon/pull/67