A first benchmark: Putting things on the dispatcher

[badboy]

This sets up criterion, a statistics-driven micro-benchmarking tool. It also adds a first simple benchmark: Launching tasks on the global dispatcher.

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Needs some docs I guess.
The way I've been running this:

On this commit:

cargo bench -p glean-core --bench dispatcher -- --save-baseline bn

Then on later commits (e.g. #3318)

cargo bench -p glean-core --bench dispatcher -- --baseline bn

results in output like this:

   Compiling glean-core v66.1.1 (/home/jer/src/mozilla/glean/glean-core)
    Finished `bench` profile [optimized] target(s) in 18.62s
     Running benches/dispatcher.rs (target/release/deps/dispatcher-b3ccfcc39ee552d0)
empty fn                time:   [61.181 ns 61.231 ns 61.285 ns]
                        change: [−23.764% −23.575% −23.390%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  5 (5.00%) low severe
  3 (3.00%) low mild
  2 (2.00%) high mild
  3 (3.00%) high severe

fn 1024                 time:   [431.57 ns 432.26 ns 432.99 ns]
                        change: [−44.413% −34.742% −23.066%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

[chutten]

What do the perf folks think?