Combine two CRC-32 checksums into one.
crc1
CRC-32 checksum for sequence A
crc2
CRC-32 checksum for sequence B
len2
Length of sequence B
Given the CRC-32 checksum 'crc1' of a sequence A and the
CRC-32 checksum 'crc2' of a sequence B of length 'len2',
return the CRC-32 checksum of A and B concatenated.