assert paranoia
move utils to tl namespace
fix includes
move id assignment to generate function

we generate fast enemy the same way, so lets do the same for
ships as well
move create & generate code to generators.h
units_arrange_in_columns now accepts range
for-each loop could be performed w/o boost::for_each
remove local enemy_generator variable, no need
unit_maybe_fire now works on range
unit_damage_all_collided now works on ranges
bounce_of_the_bounds_x now works on range
rename unit{,s}_maybe_fire
split unit_draw => units_draw & unit_draw
split unit_move => units_move & unit_move