fastjson.cc is becoming unmaintainable

`fastjson.cc` is a series of print statements that has to match the schema defined in ZTag. This is very difficult to keep up to date and reason about. At the very least, we should clean up the code so that it's not just a few huge functions. Ideally, we should find a way to make the whole process less manual. However, I'm not sure what a solution for that would like that.