Switched from CRC32 to CRC32C

Also made the CRC32C API public.  The main reason for the switch is to take
advantage of SSE4.2's support for the algorithm in the future.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Added support for SSE4.2's hardware accelerated CRC32C

To make use of this, passing CFLAGS=-msse4.2 on the configure command line is
required.  The algorithm can easily be optimized further by processing chunks
larger than bytes.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>