Sse4 integration

This code is about 20% faster than original on Haswell for bench.c.

% git co -b tmp origin/kazuho/sse4-integration
Branch tmp set up to track remote branch kazuho/sse4-integration from origin.
Switched to a new branch 'tmp'
has:/home/shigeo/Program/h2o/deps/picohttpparser% gcc -O3 -march=native picohttpparser.c bench.c -g && time ./a.out
2.156u 0.000s 0:02.15 100.0%    0+0k 0+0io 0pf+0w

% git co sse4-integration
Switched to branch 'sse4-integration'
has:/home/shigeo/Program/h2o/deps/picohttpparser% gcc -O3 -march=native picohttpparser.c bench.c -g && time ./a.out
1.661u 0.000s 0:01.66 100.0%    0+0k 0+0io 0pf+0w