Performance improvement for nearest_point_index

I was trying to slice a rather large model with honeycomb infill today. After about 20 minutes, it was still maxing out my CPU. After a bit of playing around in nytprof, I found that inlining comparable_distance_between_points (and then a few further tweaks) was a huge performance boost. Those tiny method calls and object lookups really start to add up pretty quickly.

Converting my sample .stl, I see a 20% decrease in runtime of slic3r.pl. Across the slic3r test suite (which I know isn't a performance test suite), I see a more modest 7% drop.