go/printer: remove gratuitous string/[]byte conversions
go/printer: remove gratuitous string/[]byte conversions

Cleanup and slight performance improvement (1.5%).

Before (best of 3 runs):
printer.BenchmarkPrint	      50	  47377420 ns/op

After (best of 3 runs):
printer.BenchmarkPrint	      50	  46707180 ns/op

R=rsc
CC=golang-dev
http://codereview.appspot.com/5416049

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