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/unit_tests/breath.dm

@@ -25,8 +25,7 @@
 	var/turf/open/to_fill = run_loc_floor_bottom_left
 	//Prep the floor
 	to_fill.initial_gas_mix = OPENTURF_DEFAULT_ATMOS
-	to_fill.air = new
-	to_fill.air.copy_from_turf(to_fill)
+	to_fill.air = to_fill.create_gas_mixture()
 
 	lab_rat.breathe()
 
@@ -65,8 +64,7 @@
 	var/turf/open/to_fill = run_loc_floor_bottom_left
 	//Prep the floor
 	to_fill.initial_gas_mix = LAVALAND_DEFAULT_ATMOS
-	to_fill.air = new
-	to_fill.air.copy_from_turf(to_fill)
+	to_fill.air = to_fill.create_gas_mixture()
 
 	lab_rat.breathe()