buzhash64e: with rolling hash encryption

[ThomasWaldmann]

Rather simple and very slow encryption code, needs to be tuned later IF we find this worthwhile doing.

Note:

• the encryption of a buzhash64 current sum value must only depend on the sum value and the encryption key, NOT on the position in the current stream (thus: no AES-CTR or any other cipher that does not independently encrypt blocks, this is why ECB is used here) - otherwise it would cut the chunks not only based on the content but also based on position in the stream and that would cut differently and would destroy deduplication.
• instead of encrypting just 64bit using ECB, we could collect e.g. 1024 sums, encrypt them all on in one .encrypt_many() call and then do another loop over the esums to decide where to cut. would be much less overhead than now, but needs maintaining yet another buffer efficiently.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.43%. Comparing base (5aa536d) to head (e7ad355).
⚠️ Report is 202 commits behind head on master.

Additional details and impacted files

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  Coverage   81.43%   81.43%           
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  Files          77       77           
  Lines       13515    13516    +1     
  Branches     2004     2004           
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+ Hits        11006    11007    +1     
  Misses       1853     1853           
  Partials      656      656           

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[darkk]

(As a sidenote on speed). I've found it quite interesting that fast PCG RNG (numpy's default prng) is predictable, but seed reconstruction out of 64 consecutive values is a non-trivial operation — 2¼ CPU-years of computation. However, it's still just a €108, if we assume a CPU core to cost 4 €/month.