Change implementation of remove_dir_all from recursive to an iterative version

[asder8215]

This PR essentially changes the underlying remove_dir_all implementation used by unix and I believe uefi platforms from a recursive-based approach to an iterative version. The order it removes directories and files from the recursive version should be preserved in this iterative version. I'm curious if this iterative approach could show any performance improvement over the current recursive based approach that remove_dir_all has on unix/uefi platforms.

I also made a comment on how this iterative-based approach is redundantly holding a PathBuf in the tuple (which could incur a big allocation cost if we are removing deeply nested directories). This could be optimized if we are able to retrieve a reference to the path that ReadDir struct is holding, as it contains this information in its InnerReadDir's root field; I believe this would also make the iterative approach allocate less memory than the recursive approach as we would only need the main directory/nested directories &Path values when we are removing it.

I can make an ACP for ReadDir struct root path to be accessible via a method if that would be appropriate.

[rustbot]

r? @jhpratt

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• Owners of files modified in this PR: @ChrisDenton, libs
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[asder8215]

Also, noting that this could use the std library LinkedList implementation if the resizing cost from VecDeque is undesirable.

[jhpratt]

@rustbot reroll