Allow pruning dirnames in a topdown walk to improve speed on large filesystems

[epizut]

Allowing modifying dirnames in order to improve browsing large filesystems:

os.walk

When topdown is True, the caller can modify the dirnames list in-place (perhaps using del or slice assignment),
and walk() will only recurse into the subdirectories whose names remain in dirnames;
this can be used to prune the search, impose a specific order of visiting, or even to inform walk()
about directories the caller creates or renames before it resumes walk() again.

Related issue: Add topdown kwarg to AbstractFileSystem.walk()

[ianthomas23]

Fixed by #1263.