Add a benchmark

[skywhale]

• We need to be able to compare performance of the actor system before and after the restructuring
• Ideally we also have benchmark against other actor crates in the wild

[strohel]

Perhaps one of the examples could be adapted as a simple benchmark. Sending one small message across the system to measure latency. Send a longer stream of bigger messages, measure the total time as a proxy to throughput.

Challenge: benchmark the crate itself rather than crossbeam, and OS CPU scheduler (due to the thread-based architecture).

[mcginty]

Added a simple benchmark suite in 6bbed84f7ba60243cf94df70f0a2f3ab43807e4d, but I think it's still probably flawed and could use some more attention if we want to take the numbers seriously.

[strohel]

Let's call this done, we now have various actor "shape" benches plus a pub/sub bench.