for(var/bad_code in world)

Removes a very large amount of world loops.
Adds a macro to painlessly generate a global list, and the needed code to modify the list when an object is made or deleted automatically.
Cleans up some commented out code.

code/controllers/subsystems/air.dm

@@ -35,7 +35,7 @@ SUBSYSTEM_DEF(air)
 
 	current_cycle = 0
 	var/simulated_turf_count = 0
-	for(var/turf/simulated/S in world)
+	for(var/turf/simulated/S in turfs)
 		simulated_turf_count++
 		S.update_air_properties()
 		CHECK_TICK

code/controllers/subsystems/atoms.dm

@@ -46,7 +46,7 @@ SUBSYSTEM_DEF(atoms)
 				CHECK_TICK
 	else
 		count = 0
-		for(var/atom/A in world)
+		for(var/atom/A in world) // This must be world, since this operation adds all the atoms to their specific lists.
 			if(!A.initialized)
 				InitAtom(A, mapload_arg)
 				++count

code/controllers/subsystems/xenoarch.dm

@@ -30,7 +30,7 @@ SUBSYSTEM_DEF(xenoarch)
 	. = ..()
 
 /datum/controller/subsystem/xenoarch/proc/SetupXenoarch()
-	for(var/turf/simulated/mineral/M in world)
+	for(var/turf/simulated/mineral/M in turfs)
 		if(!M.density)
 			continue