Noir::DirListing

One directory's entries and their lstat results, taken off the thread that walks the tree.

The detect walk spent most of its time in opendir, readdir, lstat and one access per directory (the shard.yml check), all on the thread that also dispatches detection. The walk submits each directory here when it pushes it and takes the listing when it pops it, so the syscalls run ahead on Noir::WorkerThreads.walk_context while everything the walk decides — pruning, filters, order — is unchanged.