Duplicate field names in @DataEnum specification interfaces

[togi]

Currently the following is allowed by the annotation processor:

@DataEnum
interface Text_dataenum {
    dataenum_case Foo(int bar):
    dataenum_case Foo(long bar);
}

However this will cause two inner classes to be generated with the same name, so the resulting class does not compile.

The annotation processor should detect duplicate case names and abort compilation with an error explaining the problem.