added crc32c() function using either optimized unrolled version, or SSE 4.2 instruction: naive rolled crc32c is 325 MB/s, unrolled/asm crc32cfast() is 1.7 GB/s, crc32csse42() is 3.5 GB/s - it is now used as default hashing function (i.e. DefaultHasher global variable)