additional refactoring, moving player type into its own directory

fcore/enemy/enemy-pair.sml

@@ -0,0 +1,11 @@
+structure EnemyPair =
+struct
+  type key = int
+  type value = EnemyType.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