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This represents yet another iteration on how `ignore` enqueues and distributes work in parallel. The original implementation used a multi-producer/multi-consumer thread safe queue from crossbeam. At some point, I migrated to a simple `Arc<Mutex<Vec<_>>>` and treated it as a stack so that we did depth first traversal. This helped with memory usage in very wide directories. But it turns out that a naive stack-behind-a-mutex can be quite a bit slower than something that's a little smarter, such as a work-stealing stack used in this commit. My hypothesis for why this helps is that without the stealing component, work distribution can get stuck in sub-optimal configurations that depend on which directory entries get assigned to a particular worker. It's likely that this can result in some workers getting "more" work than others, just by chance, and thus remain idle. But the work-stealing approach heads that off. This does re-introduce a dependency on parts of crossbeam which is kind of a bummer, but it's carrying its weight for now. Closes #1823, Closes #2591 Ref https://github.com/sharkdp/fd/issues/28