Rejecting `__eq__` early if hashes disagree

I'm not sure it's profitable for us, but it might be worth studying. The main reason I have doubts is because `freeze`s on equivalent computation lead to frequent graphs that *do* compare equal, meaning there's no benefit to an early exit that won't be taken. (And, if we were to compute the hash for this, there would be potentially substantial extra cost.)

Another option would be to
- make the hash value cache an attribute (e.g. `self._hash_value`) instead of implicit (via `@memoize_method`) and
- only use it for early exit if it's already computed (i.e. not `None`)