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>
3 files changed