Zlib:crc32 combine

Combine two CRC-32 check values into one. For two sequences of bytes, seq1 and seq2 with lengths len1 and len2, CRC-32 check values were calculated for each, crc1 and crc2. returns the CRC-32 check value of seq1 and seq2 concatenated, requiring only crc1, crc2, and len2.

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