fix issue of classFor. (causes ExceptionInInitializerError)

When I used native-gen to the eclipse-jdt jars (AST Parsers),  following error occured:
> Exception in thread "main" java.lang.ExceptionInInitializerError
>         at java.lang.Class.forName0(Native Method)
>         at java.lang.Class.forName(Class.java:259)
>         at frege.nativegen.java.Lang.eval(Lang.java:831)
> ...

"ExceptionInInitializerError" is raised when the static initializer failed.
And above dump is caused by Class.forName in function classFor.

  According to http://stackoverflow.com/a/7099453,  the method "java.lang.Class.forName" calls
static initializer 'clinit' on loading the classes, and the native-gen uses 'java.lang.Class' object
only for metainfo, thus, I think this behavior won't be necessary.
  In contrast, the "java.lang.ClassLoader.loadClass" doesn't call 'clinit', and I suppose it will be better.

Thank you for reading this.