Include PathFinder math

[kvark]

https://github.com/servo/pathfinder/blob/71623eaab312ee0c33ba8ffa70954e166b564dfb/geometry/Cargo.toml

This one uses SIMD, but it could be more difficult to hook up. There is no mint support that I can see.

[bitshifter]

I had a quick look at adding this, currently on this branch https://github.com/bitshifter/mathbench-rs/tree/pathfinder.

I started with the Euler test and ran into issues because the pathfinder vector types don't support std::ops::AddAssign which seemed like fairly basic functionality and I didn't want to rewrite the benchmark to work around this.

Perhaps I should just open an issue asking for them to add it.

[bitshifter]

I've added benchmarks for pathfinder_geometry where it made sense. It's pretty minimal compared to the other libraries so I've left it as an optional library but I'll include it's results in the README next time I update it.