exp/norm: fix benchmark bug
exp/norm: fix benchmark bug

- don't use range over string to copy string bytes
- some code simplification

R=mpvl
CC=golang-dev
http://codereview.appspot.com/5144044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go