Why `cirq` needs `Qid` class instead of just `Qudit` class?

Well, originally the idea was that we didn't want to exclude things like continuous quantum systems, and ancillary systems being used as control surfaces, and concepts like "a place where an ion might be" as opposed to the ion itself. So we went with a very generic "identifier for quantum-associated thing" name (qid).

At some point Qid gained a dimension method that returns an integer telling you how many quantum levels there are... so it really basically is a qudit now.