Shortcuts on traffic-calmed streets and private driveways

https://www.openstreetmap.org/way/1053284596 with `highway=service, access=private, service=driveway` probably shouldn't have any shortcuts. Actually, https://www.openstreetmap.org/node/10811300263 is a `barrier=gate` that should be blocking this right now, but I think we filter those out for some legacy reason... Revisit this.

https://www.openstreetmap.org/way/7997666 Willow Walk does have possible shortcuts, but there are several `traffic_calming=hump`s like https://www.openstreetmap.org/node/10278959664 along it. So maybe it'd be useful to influence the shortcut calculation somehow, or at least visually indicate them.

Both of these are related to #51 -- technically a shortcut is possible, but properties of the road segment make it much less likely.