Fast Menger Sponge for OpenSCAD

The [Menger sponge](https://en.wikipedia.org/wiki/Menger_sponge) is a 3D fractal which is more or less the 3D version of the [Sierpinski carpet](https://en.wikipedia.org/wiki/Sierpinski_carpet). In fact, each of the faces on the overall cube shape is a Sierpinski carpet.

STLs are provided for iterations from 0(plain cube) to 4.

A diagonally cut version of each is provided for ease of printing with FDM. Two diagonal prints will form the whole sponge.