add readUtf8 benchmark
add readUtf8 benchmark

SAGV/buffer,btd/buffer,nolanlawson/buffer,beni55/buffer,feross/buffer,SAGV/buffer,nolanlawson/buffer,LinusU/buffer,feross/buffer,feross/buffer,loon3/buffer,loon3/buffer,LiuDeng/buffer,LiuDeng/buffer,beni55/buffer,LinusU/buffer,btd/buffer