correct number of fruits is respawned. Fixes #28

Matching sometimes included matched fruits in the list more than once,
so they'd respawn more than once. Matching now returns a HashSet instead
of a list to avoid that. We didn't care about order anyway.

Also affects #54: with fewer fruits, the squish is much less noticeable.
avoid most vertical overlap of fruits. #28

randomize y-coordinate. Also delete some old code and add logging