[WIP] Nerf shrubs

This looks like a lot of change, but all I did was put the collision code in a loop and have the impact damage accumulate until either the obstacle breaks or the vehicle runs out of momentum.
Post-change, plowing over bushes at 20MPh reliably bashes through them, 10MPh is still iffy.

Extracting the special effect (sound and log messages) to outside that loop contributed to the mess a bit, but otherwise every impact that triggers the iteration causes a rabbitsplosion and log spew.
If someone wants to instead somehow calculate the momentum loss that would be required to smash immovable obstacles and apply that, it'd probably be cleaner, and please do so.

Can probably get better results by tweaking various numbers, but what I'd really like to do is have some test cases and our expected results written up, then we can tweak until the test cases work.