flea circus

Copperbox revision 2988.

More work on flea motion.

I think I'm happy that this code models how I want the flea motion to be controlled and the code is clear enough (although the model is simplified as it doesn't yet account for collisions).

There are a couple of intrinsics to the motion that are now captured:

* Fleas should walk left (or right) with the arrow keys pressed. After the arrow is release there is still some residual forward motion until the flea decelerates.

* Jumping should account for the speed/direction at the point of the jump key-press. During the jump - pressing arrow keys has no influence, i.e. control is like a long jump (rather than a jet-pack!).