Sample portable 2D game implemented with the `simple_2d` API of gamnit

This projects is organized in 3 modules, one per concern:

* `game_logic` defines the pure game logic without any of the display details.

* `asteronits` implements the display logic with support for keyboard input events.

* `touch_ui` adds an interface for touchscreen devices with buttons to open fire and control the ship.

# Art

* Graphics and laser sound created by Kenney.nl under CC0.
* Remote explosion sound created by NenadSimic under CC0.
* Close explosion sound created by dklon under CC-BY 3.0.