Fractals/Sets

There's an exercise about complex numbers (#718), so let's have exercises that uses those!

Have the user implement and visualise different fractals based on the following sets:
- [Julia/Fatou set](https://en.wikipedia.org/wiki/Julia_set)
- [Mandelbrot set](https://en.wikipedia.org/wiki/Mandelbrot_set)

What does this teach?
- Recursion
- Applications for complex numbers
- Visualisation methods, such as generating images from matrices and optionally generating animations
- Unittesting for software that outputs images (via visual regression testing)

Example image of a julia set:
(Source: https://commons.wikimedia.org/wiki/File:Julia_set_(ice).png)

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I think this would make for an interesting exercise. The required math/algorithm is rather simple but it would teach a lot about testing.

However, the test data would be quite unusual compared to the usual exercism tests as it would require visual regression testing or comparing large matrices. It would also be possible to just have reference images as test data and then let the user compare it to that manually. The canonical data would probably just be reference images which could be replaced by track-specific images, depending on the rendering library of choice (or of ppm images).

What are your thoughts on the idea and the required testdata?