prototypes: fix penetrations using spring forces.

The spring constant seems to be insanely large, but nothing less is
reliably working. Also reordered and slightly documented the impulse
expression, with spring forces it could happen that the impulse could be
computed in wrong direction (pushing the bodies towards each other
instead of pushing them apart), fixed that.
prototypes: it seems that subdividing physics step is not needed at all.

It didn't help with penetrations. DROP IT!
prototypes: code reordering.
prototypes: insane engine power.

Makes it possible to:

 * Do a full circle. (yay!)
 * Test physics stability with huge spinning speeds -- yes, it will
   penetrate, but after slowing down everything eventually returns to
   sane boundaries.
 * Throw everything through the roof and escape the known universe.