Optimized the game with object pooling

This is the first post-jam update of Rocketjumper!

Here we implemented a design pattern called 'object pooling'. This should not change the gameplay in any way, but should hopefully make the game run a but more smoothly by recuding lag.

Many thanks to the user "Embrace The Code" for the great suggestion :)

The browser and the Windows download versions of the game have been updated, but for anyone who wants to experience the game as it was during the game jam there is a separate download. (Although I really don't see why you would want this.)