rust-status: traverse working directory in parallel
Using `rayon` for this task ensures that we are using the same work-stealing
threadpool for everything.
This change introduces `crossbeam` as an explicit dependency, although it is
already a dependency of `rayon`. It provides better structures for
multi-threaded tasks than the stdlib.
Differential Revision: https://phab.mercurial-scm.org/D8251
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