Add some benchmarks!

One of the main focuses of this library is on making the fractal generation and rendering pipelines as fast as possible... it would be nice to have a way to automatically check if the changes in a given PR have an impact on the speed. The best way to do this is with a proper benchmarking suite. We have an approximation now, by measuring the time to generate each fractal and saving it to a file, but we could do much better.

It looks like https://crates.io/crates/criterion is a popular crate for doing this. There might be others.

Benchmarks to add:

• Inner loop of the mandelbrot set (pixel iteration)
• simulation for the DDP at a single initial condition
• (TODO: add more ideas here)