adding benchmarks

[yanns]

I'm looking into adding benchmarks to this library, inspired by https://github.com/hyperium/hyper/blob/master/benches/body.rs
The goal is to make it easier to check whether a change has a positive impact or not.

Before starting the implementation, I'd like to gather some initial feedback from the maintainers:

• do you think it's a good idea?
• if yes, do you have a preference which library should be used? (criterion, divan, other?)