See https://golang.org/cl/44150043/

««« original CL description
regexp: use sync.Pool

For machines, not threads.

Update #4720

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/44150043
»»»

TBR=golang-dev
CC=golang-codereviews
https://golang.org/cl/48190043