Explicit SSE2 vectorization of Chorba CRC method

Explicit SSE2 vectorization of Chorba CRC method

The version that's currently in the generic implementation for 32768
byte buffers leverages the stack. It manages to autovectorize but
unfortunately the trips to the stack hurt its performance for CPUs which
need this the most. This version is explicitly SIMD vectorized and
doesn't use trips to the stack.  In my testing it's ~10% faster than the
"small" variant, and about 42% faster than the "32768" variant.