add benchmark for encoding to utf8
add benchmark for encoding to utf8

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