regexp: use sync.Pool
regexp: use sync.Pool

For machines, not threads.

Update Issue 4720

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/44150043

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme