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.