Atmos init speedup, saves 4 seconds (#69697)

* Micro optimizes ssair's turf init, saving 2 seconds

Most of this is making existing operations do more legwork, or cheaper.
I did add cycle checking to ONLY init turf linking, which required
creating a new proc.
Did some horrible horrible things in said proc to save like 0.8 seconds.
I think it was worth it.

code/modules/admin/verbs/fix_air.dm

@@ -11,11 +11,10 @@
 		var/range=input("Enter range:","Num",2) as num
 		message_admins("[key_name_admin(usr)] fixed air with range [range] in area [T.loc.name]")
 		usr.log_message("fixed air with range [range] in area [T.loc.name]", LOG_ADMIN)
-		var/datum/gas_mixture/GM = new
 		for(var/turf/open/F in range(range,T))
 			if(F.blocks_air)
 			//skip walls
 				continue
-			GM.parse_gas_string(F.initial_gas_mix)
+			var/datum/gas_mixture/GM = SSair.parse_gas_string(F.initial_gas_mix)
 			F.copy_air(GM)
 			F.update_visuals()