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

crates/ignore/Cargo.toml

@@ -19,6 +19,7 @@ name = "ignore"
 bench = false
 
 [dependencies]
+crossbeam-deque = "0.8.3"
 globset = { version = "0.4.10", path = "../globset" }
 lazy_static = "1.1"
 log = "0.4.5"