Fix medusa levels

My recent commit broke medusa-3 and medusa-4, so fix those.

Make des.stair and des.ladder also accept hash-coordinate:
  des.stair("up", {8, 10});

dat/medusa-4.lua

@@ -99,7 +99,7 @@ for i=1,7 do
    des.trap()
 end
 --
-des.monster("Medusa", mx, my)
+des.monster("Medusa", place:rndcoord(1))
 des.monster("kraken", 07,07)
 --
 -- the nesting dragon