Parallelise execution

[danielcompton]

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Kibit runs in a single thread when doing its checks. This is fast enough in small projects, but in larger projects like Clojure itself, it can be a bit slow. Parallelising the checks over files, and maybe even top level forms could make the work faster. The tricky part is that we still want to report our results in file and form order, even though the results will be coming back in parallel.