HeapsIO quickstarters

Samples on how to implement customary game features with the HeapsIO API.

1. "astro shooter"
   • very close to the HeapsIO API, no custom Entity class etc.
2. "astro shooter" (performance boosted version)
   • no findAll() in the game's update() loop to gain performance
   • requires maintaining Haxe arrays instead
3. "RPG"
   • Sample on how to implement the HeapsIO h2d.Camera and separating world and UI by h2d.Scene's h2d.Layers