Change: Use bitmap for free unit ID generation. (#12165)

This improves performance of finding the next free unit number for a vehicle.

Based loosely on pool's used slot bitmap.

src/vehicle_cmd.cpp

@@ -194,6 +194,8 @@ std::tuple<CommandCost, VehicleID, uint, uint16_t, CargoArray> CmdBuildVehicle(D
 				GroupStatistics::CountVehicle(v, 1);
 				if (!(subflags & DC_AUTOREPLACE)) OrderBackup::Restore(v, client_id);
 			}
+
+			Company::Get(v->owner)->freeunits[v->type].UseID(v->unitnumber);
 		}