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

gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,gnanderson/go,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go