GameState::reward_for_player is no longer a trait function

We need GameState::rewards_for_players implemented, the library
currently doesn't need to specify for a singular player.

benches/e2e.rs

@@ -125,6 +125,19 @@ impl TicTacToe {
 
         None
     }
+
+    fn reward_for_player(&self, player: &Player) -> RewardVal {
+        if let Some(winner) = self.get_winner() {
+            if winner == *player {
+                return 1.0; // Win
+            } else {
+                return 0.0; // Loss
+            }
+        }
+
+        // Draw
+        0.5
+    }
 }
 
 impl GameState for TicTacToe {
@@ -161,19 +174,6 @@ impl GameState for TicTacToe {
         self.get_winner().is_some() || self.moves_played == 9
     }
 
-    fn reward_for_player(&self, player: &Self::Player) -> RewardVal {
-        if let Some(winner) = self.get_winner() {
-            if winner == *player {
-                return 1.0; // Win
-            } else {
-                return 0.0; // Loss
-            }
-        }
-
-        // Draw
-        0.5
-    }
-
     fn rewards_for_players(&self) -> HashMap<Self::Player, RewardVal> {
         HashMap::from_iter(vec![
             (Player::X, self.reward_for_player(&Player::X)),