net/http: add Hihack benchmark
net/http: add Hihack benchmark

Notably, to show allocs. Currently: 11766 B/op, 21 allocs/op,
at least one alloc of which is in the benchmark loop itself.

R=golang-dev, jnewlin
CC=golang-dev
https://codereview.appspot.com/40370057

wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old