Improve generated crossing geometry near sharp angles

[dabreegster]

This case is fixed in a simple way, but it reveals more problems:

https://a-b-street.github.io/speedwalk/#19.95/52.7923616/13.2608191

We really need to handle the angle of roads better, or look for the "corner" in the generated sidewalks to place things better. (This might introduce more dependency of bulk operation steps, because then we have to generate sidewalks before crossing nodes.)

Originally posted by @dabreegster in #83