- no need to make copies in cases of aliases
- removed deprecated internal shift functions
- minor unrelated simplifications

This change improves pidigits -s -n10000 by almost 20%:

user 0m6.156s (old)
user 0m4.999s (new)

(pidigits -s -n20000 goes from ~25s to ~19s)

R=rsc
CC=golang-dev
https://golang.org/cl/1149041