Moving squaredDistanceToSegment to the coordinate package
Let closestOnSegment return nothing more than a coordinate

Since we do not use the 'along' property anywhere, and the
resulting array returned by closestOnSegment could cause trouble
when working with 3d coodinates, now only the closest point is
returned, and the squared distance to the segment is calculated
by squaredDistanceToSegment instead.
Add tests
Merge pull request #1225 from ahocevar/closest-on-segment

squaredDistanceToSegment and footOfPerpendicularToSegment