Adds the Pool type and docs, and use it in fmt.
This is a temporary implementation, until Dmitry
makes it fast.

Uses the API proposal from Russ in http://goo.gl/cCKeb2 but
adds an optional New field, as used in fmt and elsewhere.
Almost all callers want that.

Update #4720

R=golang-dev, rsc, cshapiro, iant, r, dvyukov, khr
CC=golang-dev
https://golang.org/cl/41860043