The existing implementation of flag values with fmt package uses
more memory and works slower than the implementation with strconv
package.

Change-Id: I9e749179f66d5c50cafe98186641bcdbc546d2db
Reviewed-on: https://go-review.googlesource.com/28914Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>