bbox_collision_detector: pass ObjectIdentifier as const& where possible
bbox_collision_detector: pass ObjectIdentifier as const& where possible

This may make things a tad faster for nontrivial ObjectIdentifiers
like boost::variants with many-byte tile_locations inside.

elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,elidupree/Lasercake,elidupree/Lasercake,idupree/Lasercake,idupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake,idupree/Lasercake,Lasercake/Lasercake,elidupree/Lasercake,idupree/Lasercake,Lasercake/Lasercake