constant time Ord and PartialOrd implementations #267
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Fixes #202.
This does not follow the recommended path of utilizing subtle as it does not provide any helpers to do constant time ordering on containers.
The overall method is to compare each u32 in the hashes, turn each of those into -1, 0, or 1, accumulate them into a single signed word, and then finally compare that word to 0.
The optimized object code on x86-64 is somewhat large but appears to be constant time - no jumps, early returns, or anything like that. This is, of course, not a guarantee that it will never face optimization in hardware or software in the future.