Support non-primitive parameters

[domn1995]

As a developer, I want my union definitions to support non-primitive parameters, so that I can build more complex and useful unions.

I should be able to do something like this:

using System;

[Union]
interface IResult
{
    void Created(Guid id);
    void Failure(Exception error);
}

This currently fails to compile because the imports are not part of the generated code.