CIF state annotations: ensure all state annotations within an automaton are different

UPDATE: This constraint got removed again in #756 (closed).

Original description

From !712 (comment 1519430):

One thing I realized is that we allow duplicate @state annotations. Probably both between different locations and within one location, I am guessing. Not sure that's a problem to be reported.

From !712 (comment 1520244):

I hadn't considered that yet. But, checking that means evaluating all arguments, making an argument name to value map, and comparing those maps for all state annotations, using value equality. And all the while, all these maps need to be kept in memory. That does sound expensive (CPU and memory wise). I think I'll skip that for now. If we ever find that useful, we can always add it later.

From !712 (comment 1521531):

I realized you just need a set of state annotations with a nice equality hashing function, similar to what #542 (closed) did. Doesn't need to be done now, I'd say.