strings: Replacer is safe for concurrent use
strings: Replacer is safe for concurrent use

LGTM=r
R=golang-codereviews, bradfitz, r
CC=golang-codereviews
https://codereview.appspot.com/109220044

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go