ignore: use work-stealing stack instead of Arc<Mutex<Vec<_>>>

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

CHANGELOG.md

@@ -8,6 +8,11 @@ Unreleased changes. Release notes have not yet been written.
   `rg -B1 -A2`. That is, `-A` and `-B` no longer completely override `-C`.
   Instead, they only partially override `-C`.
 
+Performance improvements:
+
+* [PERF #2591](https://github.com/BurntSushi/ripgrep/pull/2591):
+  Parallel directory traversal now uses work stealing for faster searches.
+
 Feature enhancements:
 
 * Added or improved file type filtering for Ada, DITA, Elixir, Fuchsia, Gentoo,