Design flaw in Rest_collision

[address@hidden]

Hey team,

I finally had a couple hours to get back into LilyPond development and, as 
often happens after a pause, I immediately ran into something that I hadn't 
spotted before and that needs fixing.

If a Rest being managed by a RestCollision, a call to Grob::extent with any 
common refpoint other than the Rest itself will trigger at some point 
Rest_collision:calc_positinioning_done which itself looks up Rest extents at 
line 168.  This is a circular dependency.

If I'm reading this wrong then let me know, but if not, it needs to be fixed.  
Otherwise, getting a rest's extent can randomly lead to circular dependencies.

Cheers,
MS