Add a benchmark comparing task spawning vs in-line joining

[yoshuawuyts]

We should anticipate that when we put out the futures-concurrency RFC that members of the community may ask about which benefits e.g. join or merge provide over abstractions like task::spawn or select! {}. There are several benefits, including integration with debuggers, ease of use, documentation, structured concurrency, and overall correctness. But also, and this is relevant to the recent work we've been doing: performance!

We should author a benchmark comparing e.g. tokio::spawn and tokio::spawn_local against {tuple,array,vec}::join. We may want to implement #82 for futures as part of this, so we can control the number of wakes and don't exclusively perform synchronous work. But it'll be interesting to see the performance delta between the two.