Integrate with validator libraries

[lfn3]

One feature that was requested was using a Java validator library to constrain the input that a generator produces. Might have to be trial and error rather than being able to parse the code, unless there's some concrete validations included in the library that we can piggy-back off.

Need to find said libraries, get an idea of their absolute and relative popularity, and some examples of their use before I can consider tackling this one.