Walls of lava

Add "walls of lava", basically lava which blocks vision and
require a bit more than just levitation or flight to move through.

No levels use this yet, as testing isn't thorough enough.

src/nhlua.c

@@ -338,6 +338,7 @@ const struct {
                 { '}', MOAT },
                 { 'P', POOL },
                 { 'L', LAVAPOOL },
+                { 'Z', LAVAWALL },
                 { 'I', ICE },
                 { 'W', WATER },
                 { 'T', TREE },