add falling_enemies type to game (these enemies can be picked up and thrown), and thread fallingEnemies through enemy.sml to set up scaffolding for them

fcore/game-update.sml

@@ -2,8 +2,16 @@ structure GameUpdate =
 struct
   fun update (game, input) =
     let
-      val {player, walls, wallTree, platforms, platformTree, enemies, graph} =
-        game
+      val
+        { player
+        , walls
+        , wallTree
+        , platforms
+        , platformTree
+        , enemies
+        , graph
+        , fallingEnemies
+        } = game
 
       val enemyTree = Enemy.generateTree enemies
       val player = Player.runPhysicsAndInput (game, input, enemyTree)
@@ -11,7 +19,7 @@ struct
       val projectiles = #projectiles player
       val projectileTree = Projectile.generateTree projectiles
 
-      val enemies = Enemy.updateEnemyList
+      val (enemies, newFallingEnemies) = Enemy.updateEnemyList
         ( Vector.length enemies - 1
         , enemies
         , projectiles
@@ -23,7 +31,10 @@ struct
         , player
         , graph
         , []
+        , []
         )
+
+      val fallingEnemies = Vector.fromList newFallingEnemies
     in
       { player = player
       , walls = walls
@@ -32,6 +43,7 @@ struct
       , platformTree = platformTree
       , enemies = enemies
       , graph = graph
+      , fallingEnemies = fallingEnemies
       }
     end
 end