Summary
-------

Mark some ClassDesc methods, whose implementations are now specialized into subclasses in the sealed hierarchy, as abstract.

Problem
-------

ClassDesc is getting specific implementations; some previous default implementations are better converted to non-concrete abstract methods and be overridden in implementations instead.

The specification of `displayName` is too restrictive; there are other human-readable options for class or interface names such as package abbreviations, but the current spec of `displayName` forbids such implementations.

Solution
--------

Make these methods in `ClassDesc` abstract (non-default):

 - `arrayType()`
 - `arrayType(int)`
 - `displayName()`

Relax the implementation specification on `displayName()`.

Specification
-------------

```
-    default ClassDesc arrayType() {
+    ClassDesc arrayType() {
-    default ClassDesc arrayType(int rank) {
+    ClassDesc arrayType(int rank) {
-     * Returns a human-readable name for the type described by this descriptor.
-     *
-     * @implSpec
-     * <p>The default implementation returns the simple name
-     * (e.g., {@code int}) for primitive types, the unqualified class name
-     * for class or interface types, or the display name of the component type
-     * suffixed with the appropriate number of {@code []} pairs for array types.
-     *
-     * @return the human-readable name
+     * {@return a human-readable name for this {@code ClassDesc}}
      */
-    default String displayName() {
+    String displayName() {
```