[code-reflection] RFR: Enable pattern matching for JavaType hierarchy

[Maurizio Cimadamore]

The `JavaType` hierarchy has now several types under it:

* `ClassType`
* `ArrayType`
* `PrimitiveType`
* `TypeVarRef`
* `WildcardType`

All these types are `public`, except `PrimitiveType`. This limits applicability of pattern matching.

In addition, `JavaType` defines a bunch of predicates:

* `isClass()`
* `isArray()`
* `isPrimitive()`

This set of predicates is now incomplete, as it doesn't support all the possible leaves in the type hierarchy.

Overall, it would be much better to just lean into pattern matching more, and drop the predicates.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/64/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=64&range=00
  Stats: 99 lines in 7 files changed: 0 ins; 91 del; 8 mod
  Patch: https://git.openjdk.org/babylon/pull/64.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/64/head:pull/64

PR: https://git.openjdk.org/babylon/pull/64