Basalt turfs (#3985)

changes:

Adds three new turf types: lava (separate from the existing lava which is bound to the flooring system), basalt (a child of asteroid turfs), and chasm open spaces.
canSmoothWith lists on asteroid turfs are now in the form of a shared global list for performance reasons.
'Ash' turfs have been repathed to /turf/simulated/asteroid/ash. Base asteroid turf type should not be used directly anymore; maps have been updated to new path.
Airless openspace, chasms, and airless chasms now have map-time debug icons.
No player-visible changes, just prepwork for future things and performance tweaks.

Sprites from TG.

code/modules/multiz/turf.dm

@@ -157,6 +157,31 @@
 	oxygen = 0
 	nitrogen = 0
 	temperature = TCMB
+	icon_state = "opendebug_airless"
+
+/turf/simulated/open/chasm
+	icon = 'icons/turf/smooth/chasms_seethrough.dmi'
+	icon_state = "debug"
+	smooth = SMOOTH_TRUE | SMOOTH_BORDER | SMOOTH_NO_CLEAR_ICON
+	no_mutate = TRUE
+	name = "hole"
+
+/turf/simulated/open/chasm/airless
+	oxygen = 0
+	nitrogen = 0
+	temperature = TCMB
+	icon_state = "debug_airless"
+
+/turf/simulated/open/chasm/airless/Initialize()
+	. = ..()
+	icon_state = "Fill"
+
+/turf/simulated/open/airless/chasm/get_smooth_underlay_icon(mutable_appearance/underlay_appearance, turf/asking_turf, adjacency_dir)
+	underlay_appearance.icon = 'icons/turf/basalt.dmi'
+	underlay_appearance.icon_state = "basalt"
+	if (prob(20))
+		underlay_appearance.icon_state += "[rand(0,12)]"
+	return TRUE
 
 /turf/simulated/open/post_change()
 	..()