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/mkroom.c

@@ -1012,6 +1012,9 @@ cmap_to_type(int sym)
     case S_water:
         typ = WATER;
         break;
+    case S_lavawall:
+        typ = LAVAWALL;
+        break;
     default:
         break; /* not a cmap symbol? */
     }