Add subsystem for safely spawning atoms at roundstart

This system is called just before initialize and does not use a world
loop, so it is safe to spawn and create new atoms.

Any atom can inherit this behaviour by overriding the
spawn_atom_to_world proc and spawning atoms there at roundstart

Make sure you register the atom in question to the list of spawner atoms
by doing SSobj.atom_spawners += src, and then remove when you are done
spawning objects

code/controllers/subsystem/objects.dm

@@ -9,6 +9,7 @@ var/datum/subsystem/objects/SSobj
 	name = "Objects"
 	priority = 12
 
+	var/list/atom_spawners = list()
 	var/list/processing = list()
 	var/list/currentrun = list()
 	var/list/burning = list()
@@ -17,6 +18,7 @@ var/datum/subsystem/objects/SSobj
 	NEW_SS_GLOBAL(SSobj)
 
 /datum/subsystem/objects/Initialize(timeofday, zlevel)
+	trigger_atom_spawners()
 	setupGenetics()
 	for(var/V in world)
 		var/atom/A = V
@@ -26,6 +28,12 @@ var/datum/subsystem/objects/SSobj
 		CHECK_TICK
 	. = ..()
 
+/datum/subsystem/objects/proc/trigger_atom_spawners(zlevel)
+	for(var/V in atom_spawners)
+		var/atom/A = V
+		if (zlevel && A.z != zlevel)
+			continue
+		A.spawn_atom_to_world()
 
 /datum/subsystem/objects/stat_entry()
 	..("P:[processing.len]")
@@ -57,4 +65,5 @@ var/datum/subsystem/objects/SSobj
 /datum/subsystem/objects/proc/setup_template_objects(list/objects)
 	for(var/A in objects)
 		var/atom/B = A
-		B.initialize()
+		B.spawn_atom_to_world()
+		B.initialize()

code/game/atoms.dm

@@ -358,6 +358,10 @@ var/list/blood_splatter_icons = list()
 	. = ..()
 	sleep(1)
 
+//This is called just before maps and objects are initialized, use it to spawn other mobs/objects
+//effects at world start up without causing runtimes
+/atom/proc/spawn_atom_to_world()
+
 //This will be called after the map and objects are loaded
 /atom/proc/initialize()
 	return
@@ -385,4 +389,4 @@ var/list/blood_splatter_icons = list()
 		if(istype(R, /datum/reagent/consumable))
 			var/datum/reagent/consumable/nutri_check = R
 			if(nutri_check.nutriment_factor >0)
-				M.reagents.remove_reagent(R.id,R.volume)
+				M.reagents.remove_reagent(R.id,R.volume)

code/modules/awaymissions/corpse.dm

@@ -27,7 +27,9 @@
 	log_game("[user.ckey] became [mob_name]")
 	create(ckey = user.ckey)
 
-/obj/effect/mob_spawn/initialize()
+/obj/effect/mob_spawn/spawn_atom_to_world()
+	//We no longer need to spawn mobs, deregister ourself
+	SSobj.atom_spawners -= src
 	if(roundstart)
 		create()
 	else
@@ -35,6 +37,8 @@
 
 /obj/effect/mob_spawn/New()
 	..()
+	//Add to the atom spawners register for roundstart atom spawning
+	SSobj.atom_spawners += src
 	if(instant)
 		create()
 	else