Profile percentage of game state copying

[peter1591]

In current implementation,
before applying each action (i.e., play-card, attack, hero-power, or end-turn)
the game state is saved on stack
This game state is restored if the action is actually an invalid action and make the application failed.

We can make the game state to be copy-on-write,
and support the fast response for those methods which are probably invoked when applying an invalid action

Since there are many discussions on the efficiency of copy-on-write data structures,
it's better to delay after we've done some profiling.

[peter1591]

The underlying reason is that the chance leading to an invalid state should be quite low. Move to another issue to optimize this at the final stage.