speed up serde2's json serialization to 180MB/s
speed up serde2's json serialization to 180MB/s

JohnHeitmann/serde,csmulhern/serde,eefriedman/serde,tempbottle/serde,jmesmon/serde,oli-obk/rust-serde,sidred10/serde,Byron/serde,thorbenk/serde,skade/serde,Ticki/serde,3Hren/serde,infinityb/serde,pcwalton/serde,Hoverbear/serde,serde-rs/serde,erickt/serde