Attribute Macro validators

#[validators]

The user would do:

#[validators(state = TaskState, machine = TaskMachine)]
impl DbData {
    fn is_new(&self /*, ..TaskMachine fields*/) -> Result<(), statum::Error> { ... }
    fn is_in_progress(&self /*, ..TaskMachine fields*/) -> Result<DraftData, statum::Error> { ... }
    fn is_complete(&self /*, ..TaskMachine fields*/) -> Result<(), statum::Error> { ... }
}

We parse the impl block, find all is_* fns, build a wrapper enum, and build a to_machine(/*..TaskMachine fields*/) method.