Greatly improve the equality detection of TurtleWorlds

That is, merge lines that are in line with each other before testing equality.

You can now walk half of a segment, the half will be removed. You can
walk to your destination in large amount of very small lines that are
all depicting a long line, parts will be merged together. I hope that
now, that damn world equality detection will not return false negative
anymore!

(fix #338) (fix #339), and no, they were not exactly duplicate bugs :)