pidigits: ~10% performance win by using adds instead of shifts
pidigits: ~10% performance win by using adds instead of shifts

user time for pidigits -s -n=10000:
6.466s w/ adds
7.138s w/ shifts

R=rsc
CC=golang-dev
http://codereview.appspot.com/1021041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go