Replace core_bench with benchmark

[Leonidas-from-XIV]

core_bench is nice but the amount of dependency issues and build failures it causes via its core_unix dependency is draining a lot of maintenance time trying to get it to work again so this replaces it with a simpler library.

The output is a bit less fancy, but it should allow for similar measurements.

[dinosaure]

Sorry to come after your great work but you should take a look on bechamel which can provides a nice output (with notty) and/or an HTML report 👍. It's actually used for many things for Mirage projects.

[Leonidas-from-XIV]

Thanks @dinosaure. @patricoferris pointed me at bechamel before but it looked rather daunting and without a simple CLI option (requiring either notty (which itself has a spotty maintenance record) or a browser).

I would be in support of migrating to bechamel at some time in the future, but for now I just wanted to avoid the headache and benchmark didn't require any additional dependencies. That also meant that I had to cut out the support for the visualization of the data via ocurrent-bench for now, since maintaining it was more trouble than it brought in in benefits.