strings: implement a faster byte->string Replacer

This implements a replacer for when all old strings are single
bytes, but new values are not.

BenchmarkHTMLEscapeNew   1000000   1090 ns/op
BenchmarkHTMLEscapeOld   1000000   2049 ns/op

R=rsc
CC=golang-dev
https://golang.org/cl/5176043