Faster memory comparison in C

A few notes:

1. memcmp is in <string.h>

2. char is not the same as unsigned char

3. Compilers like to remove loops. Notice how there are no labels, and no branches, in main here: https://godbolt.org/g/DCTBb7 .

4. There are benchmarks of various methods: http://0x80.pl/articles/simd-strfind.html .