Don't serialize GridQubit hash when pickling

[maffoo]

When hashing Qid objects we were including the Qid class, but class hashes are not stable between restarts of the interpreter. This caused #3777 where the hash of unpickled GridQubit objects disagrees with other GridQubits to which they compare equal. Here we fix this by hashing based on the class name rather than the class object itself.

Fixes #3777

[mpharrigan]

Are string hashes stable :)

[maffoo]

@mpharrigan, that is a very good question :-)

I also noticed the tests were failing because we specifically do not want the hashes to depend on the particular Qid class, since, e.g. GridQubit(0, 0) and GridQid(0, 0, dimension=2) compare equal so should hash to the same thing. I've switched instead to just implementing the __getstate__ and __setstate__ functions used when pickling to skip storing the hash and recompute it when unpickling. Also added a test that this works as intended with pickle.

[dabacon]

Looks like there are also other places where the hash of the class is used, like Moment?

[maffoo]

I think hash of a class is fine; we just have to avoid serializing hashes when e.g. pickling, because the hashes are not stable from one run of the python interpreter to the next. I'll change the issue title to reflect this.

[mpharrigan]

awesome! Thanks for doing the fix