[code-reflection] RFR: Cleanup JavaType factories

[Maurizio Cimadamore]

This PR is a biggie cleanup of the various `JavaType` classes.

Perhaps the biggest contribution of this PR is to cleanup the factories in `JavaType`. The new factories are as follows:
* a factory to create a type from a `ClassDesc`
* a factory to create a type from a `j.l.r.Type`
* a couple of factories for creating parameterized types, which work on `JavaType` instances
* a couple of factories for array types
* a couple of factories for wildcard types
* a factory for type variables

This resolves the non-orthogonality found in the current set of factories, where one can take a `JavaType` and can, simultaneously, parameterize it, and turn it into an array type. Now the factories for these two operations are distinct.

The new factories also clarify the relationship between `JavaType`, `ClassDesc` and `j.l.Type` - that is:

* a type can be built from a nominal descriptor, using `type(ClassDesc)`
* a type can turned into a nominal descriptor, using `toNominalDescriptor()`
* a type can be built from a reflective type mirror, using `type(Type)`
* a type can turned into a nominal descriptor, using `resolve(Lookup)`

These operations are symmetric - that is, you can start from a reflective type mirror, build a `JavaType` form it, then resolve that into a `Type` and check that you get back where you started. I've enhanced `JavaTypeTest` to check round trip for both `ClassDesc` and `Type`.

Note that the `JavaType` can be associated with complex reflective mirrors, e.g. beyond just `Class<?>`. For this reason, existing uses of `JavaType::resolve` in `BytecodeGenerator` and `Interpreter` which were expecting to obtain a `Class` from a `resolve` call have been rewritten to use the idiom `type.erasure().resolve(...)`.

As part of this PR I've also added javadoc for all the methods/fields under the `JavaType` hierarchy that did not have one, and expanded existing docs.

I've added several tests to make sure that all the conversions work as expected, and I have verified the examples (by opening their files in the IDE and make sure there's no error, I did not build).

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Commit messages:
 - Update test/jdk/java/lang/reflect/code/TestErasure.java
 - Remove whitespaces
 - Add test for bi-directional conversion to/from ClassDesc/Type
 - Resolve types to reflective type mirrors
 - Add more javadoc
 - Add PrimitiveType::isVoid
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/71/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=71&range=00
  Stats: 576 lines in 26 files changed: 387 ins; 53 del; 136 mod
  Patch: https://git.openjdk.org/babylon/pull/71.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/71/head:pull/71

PR: https://git.openjdk.org/babylon/pull/71