This is the Replace I suggested in the review of CL 1114041.
It's true that we already have

	regexp.MustCompile(regexp.QuoteMeta(old)).ReplaceAll(s, new)

but because this Replace is doing a simpler job it is
simpler to call and inherently more efficient.

I will add the bytes implementation and tests to the
CL after the strings one has been reviewed.

R=r, cw
CC=golang-dev
https://golang.org/cl/1731048