Map grid segfault fix in map::displace_vehicle
formatting map::vehmove() so that i can read it

before taking a stab at rewriting it.
another functionally identical change,

adding of_turn to vehicles, to indicate how much
of their per-turn travel remains.
reworking wrapped_vehicle

to not use a more generic superclass, because i don't
intend to refer to a vehicle as an 'item'
overhaul checkpoint:

map::get_vehicles() with no args now gets all vehs.
vehmove() using get_vehicles() instead of iterating over each submap.
commented code for me to refer to and you to be anoyed by.
made wrapped_vehicles aware of submap.

used get_vehicles to de-indent a couple levels of vehmove.
more vehmove refactoring.

it's now the case that veh movement is
 sort of handled in parallel.
Too bad you hardly ever see multiple
 vehicles moving at once. (so far...)
more refactoring

an enum for collision types, and a collision struct.
nothing really, a target & subtarget entry for

collision structs, but nothing's done with it yet.
removing vehicle 'moves',

vehicles' of_turn  property replaces it,
 which tracks time instead of distance
mechanism for vehicles to carry leftover

vehproceed time to the next turn.
nerfing initial reverse thrust.

braking was confused with thrust because thrust dir
wasn't as negative as 0.

This goes as far as to make reverse less powerful than forward accel.

Also tweaking stoppage threshold so that bicycles can go in reverse.
making vehicle collisions slightly less ridiculous

hitting a bicycle with a cargo truck will make both vehs move
simultaneously. currently it just will move forward instead
of conserving momentum.

no damage involved yet.
fixing previous nerf to reverse thrusting.
modeling apocalyptic vehicle collisions with a

horrifying mixture of roguelike dynamics, bad physics, roguelike
geometry, and trigonometry.

still no vehicle damage.