flag: use strconv instead of fmt in values' String funcs

flag: use strconv instead of fmt in values' String funcs

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/28914
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>