refactor falling_enemy to be contained in FallingEnemyMap, instead of a plain vector

fcore/enemy/falling-enemy-pair.sml

@@ -0,0 +1,11 @@
+structure FallingEnemyPair =
+struct
+  type key = int
+  type value = EnemyType.falling_enemy
+
+  fun l (a: int, b: int) = a < b
+  fun eq (a: int, b: int) = a = b
+  fun g (a: int, b: int) = a > b
+
+  val maxNodeSize = 8
+end