Allow function instance instead of symbol

[facundoolano]

The reason why a symbol is required is that it would break the save/load serialization of the game state if function values are used instead.

Look into making a serializer that knows how to do it?

[facundoolano]

This could maybe be achieved by reimplementing the way save/load work. Instead of trying to save the state, keep track of all inputs of the user, and reproduce them on each load (without showing the actual output).