go/printer: cleanup more string/byte conversions
go/printer: cleanup more string/byte conversions

Slight slow-down for printer benchmark (-0.7%) before
applying CL 5416049 (which will wash it out). Code is
cleaner and simpler.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5417053
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go