RFC: Promisify benchmark harness

It would be great to have native support for promises in the harness so that the benchmark can be used in Observable. https://next.observablehq.com/@kgryte/stdlib-writing-benchmarks describes how to create the promisified harness but I propose to add this code to stdlib directly.

Start with the linked Observable benchmark guide, then locate the existing benchmark harness in the repository and compare it with the promisified approach described there. Done means the harness has native promise support and can be used for Observable benchmarks; the issue names no specific files or tests to run.

Written by the indexing model from the issue text.