Implement _stablehash function for @cache keys

[lapp0]

What behavior of the library made you think about the improvement?

Currently @cache relies on __getstate__, however it's not obvious when implementing this method that it should result in a regularized object which has a hash which is consistent with equivalent objects. Serialization doesn't necessitate an equivalent representation for equivalent objects as @brandonwillard pointed out in #929 (comment)

Further, cloudpickle doesn't guarantee determinism cloudpipe/cloudpickle#453

How would you like it to behave?

To ensure we avoid cache misses, we should first call cache_key._stablehash(), with __getstate__() as a fallback.