strings: delete Runes, Bytes

gofmt -w -r 'strings.Bytes(a) -> []byte(a)' src/cmd src/pkg test/bench
gofmt -w -r 'strings.Runes(a) -> []int(a)' src/cmd src/pkg test/bench
delete unused imports

R=r
CC=golang-dev
https://golang.org/cl/224062