arti-bench: summarize statistics with a new `Statistic` type (#298)

etarequested to merge
eta/arti:eta/298 into main

This implements the proposal from #298 (closed), making the BenchmarkResults type be made out of a bunch of new Statistic types (which summarize the mean, median, range, and standard deviation of an arbitrary value) instead of overloading TimingSummary for this purpose.

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