Make crc64 go faster

The current crc64 algorithm is a variant of the Redis implementation.
Change it to a variant of the Adler implementation as described
at https://matt.sh/redis-crcspeed

Test program at https://github.com/PeeJay/crc64-compare
Filesize: 1.1G
Asking crc64-redis to sum "/media/peejay/BTRFS/1/ubuntu-14.04.5-desktop-amd64.iso"...
Asking crc64-adler to sum "/media/peejay/BTRFS/1/ubuntu-14.04.5-desktop-amd64.iso"...
Redis CRC-64: f971f9ac6c8ba458
Adler CRC-64: f971f9ac6c8ba458
Adler throughput: 1659.913308 MB/s
Redis throughput: 437.284661 MB/s
Adler is 3.79x faster than Redis

Signed-off-by: Paul Jones <paul@pauljones.id.au>

include/crucible/crc64.h

@@ -3,6 +3,7 @@
 
 #include <cstdint>
 #include <cstdlib>
+#include <cstring>
 
 namespace crucible {
 	namespace Digest {