width: implement Span interface Change-Id: I73658f436dda909986b6a847c54945ec85a39744 Reviewed-on: https://go-review.googlesource.com/28133 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>