runes: added spanner implemenation Change-Id: Ifa7d61c682b2347ea6111910c0955ec2a2501930 Reviewed-on: https://go-review.googlesource.com/29185 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>