Speed up chunkcopy and memset

Speed up chunkcopy and memset

This was found to have a significant impact on a highly compressible PNG
for both the encode and decode.  Some deltas show performance improving
as much as 60%+.

For the scenarios where the "dist" is not an even modulus of our chunk
size, we simply repeat the bytes as many times as possible into our
vector registers.  We then copy the entire vector and then advance the
quotient of our chunksize divided by our dist value.

If dist happens to be 1, there's no reason to not just call memset from
libc (this is likely to be just as fast if not faster).