Identifying JDK Value Class Candidates

As long as value classes are a preview feature, they can’t generally be adopted in the JDK libraries (which are not compiled with `--enable-preview`). However, we felt it was important for this JEP to provide users with a “starter kit” of some value classes out of the box, and to ease into any migration challenges for popular classes, so we identified a handful of JDK classes that the JEP can treat as value classes when preview features are enabled.