Fixes corrosive acid, makes most important things unacidable

Makes pretty much anything important to the crew unacidable, fixes how acid interacts with floors.

code/game/objects/structures/cliff.dm

@@ -33,6 +33,7 @@ two tiles on initialization, and which way a cliff is facing may change during m
 	opacity = FALSE
 	climbable = TRUE
 	climb_delay = 10 SECONDS
+	unacidable = TRUE
 	block_turf_edges = TRUE // Don't want turf edges popping up from the cliff edge.
 	plane = TURF_PLANE