pidigits: Remove TODO, minor improvements
pidigits: Remove TODO, minor improvements

I tried adding a Scale method to big.Int and it didn't make any noticeable
difference, so I'm removing my TODO.

Also got rid of a few obvious allocations that I missed earlier for a modest
improvement (~5%).

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/2711043

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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