Add benchmark plots (fixes #23)

[mre]

This adds some simple benchmark plots to the REAME.md.
I chose the comic style on purpose:

The rough, seemingly hand drawn nature of the graph provides a visual hint as to the imprecision of the results.

(https://www.chrisstucchio.com/blog/2014/why_xkcd_style_graphs_are_important.html)

The plots will be created every time we run make bench-all. We could also hide this behind a separate make target, but I think it's better to have both, the benchmark and the plotting, in one place.

[mre]

I'm surprised that we lag behind in one of the simpest benchmarks: serializing and deserializing an array with just True values in them. This should be super fast and I'm confused why it's not. Will need to do some more investigations.