Make the bench_struct map test more complicated, 4373ns vs 5355ns
Make the bench_struct map test more complicated, 4373ns vs 5355ns

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