Add benchmark for Magnus

[matsadler]

Hi!

I saw your comment on reddit, and thought I'd take a go at adding Magnus to the set of libraries covered by these benchmarks.

I didn't quite get everything running in my environment, and I didn't want to bombard you with a bunch of irrelevant changes, so this is just the stuff to add Magnus.

Despite not being able to compare against the full set of the other libraries, I don't think Magnus does great at this particular benchmark. I think I spotted a potential for improvement in Magnus, so there might be another PR in the future to bump the version when I get the opportunity to make that improvement and release a new version.

I'd be interested if this makes a difference to your conclusions.

Thanks!

[matsadler]

I've added a commit to bump Magnus to the latest version. With this I'm actually seeing Rust + Magnus coming in at slightly faster than C.