Added a new state to the game.

This is a "dying" state. Cells go from alive to dying. However, this is
merely graphical. Dying cells are not considered alive, so they don't
affect the rules at all. (If they're considered alive, the simulation
stays in a random state.)

To do this:

* I defined a new bounded, enum data type for the cell state.
* I had to use an unboxed vector.
* I augmented the display and generation rules to account for this.