Add a benchmark

[bnjbvr]

While looking at #120, I figured we'd need some kind of benchmarking, to understand the impact of such a change on performance. Rather than synthetic benchmarks running the regexp, I'm thinking of having a collection of real-world Rust projects, pinned to specific commits, so it's possible to run and time cargo-machete on each of them.

This would likely take the form of a script, that can be manually run, and not an automated, continuous benchmarking on every commit (although if someone wanted to provide that, it sure would be nice!).