`dunai`: Create benchmark

[ivanperez-keera]

One of the main reasons for dunai to have changed very little, and for so many PRs to take so long to integrate, is trying to make sure that we do not seriously impact performance or break the API when we do that.

We need a series of benchmarks for dunai that allow us to 1) measure performance, and 2) compare performance, both to prior implementations and, potentially, to other FRP implementations. Of course, if they are very different, that may be really hard and need to be addressed separately.

I'm creating this issue to document this line of work and have a way to track progress in this direction.

This is also relevant for: #233, as well as to compare Yampa and bearriver.