scandir, a better directory iterator and faster os.walk()

scandir() is a generator version of os.listdir() that returns an iterator
over files in a directory, and also exposes the extra information most
operating systems provide while iterating files in a directory (such as
type and stat information).

This module also includes a version of os.walk() that uses scandir()
to speed it up significantly.

NOTE: If you're using Python version 3.5+, os.scandir() and the speed
improvements to os.walk() are already available in the standard library.