Simplify walkPast and fRelation

1. Cut back on $AS use in f* passages some more, and don't set it from walkPast.
2. Accept the desired partner as an argument to fRelation. Remove the $relation and $partner global variables. This simplifies all of the calling locations considerably (we usually have a reference to the best partner slave already, or can get it very easily), and it gives us more flexibility going forward since the scene is not responsible for choosing one at random.
3. Remove the flag mechanism and all of the old passage-scene stuff from walkPast.
4. Reduce the scope of partner in walkPast, since only one function needs it now.
5. Use the span reference rather than an explicit ID for replacing the walkPast links with the scene.