Compile-time representation of Java type system.

These classes are used as TypeT and ClassDeclT of the model parameterization. This implementaion is designed to be capable of representing pre-existing classes (such as java.lang.String) as well as the generated classes (represented as JDefinedClass.)

## Handling of Primitive Types

Primitive types have two forms (int and Integer), and this complicates the binding process. For this reason, inside the front end, we always use the boxed types. We'll use the unboxed form only in the back end when we know the field doesn't need to represent the null value.