Benchmark for new io_uring code

## TODO

- [ ] Implement simple benchmarking
- [ ] Put benchmarking guidelines back into `README.md` (see [`README.md` from this commit](https://github.com/JackKelly/light-speed-io/blob/39f6b3d29784f3ef9e06a9812c675f7de62a8ca3/README.md) for the old benchmarking instructions).