Proposed patch adds support for file descriptors in os.scandir() and implements os.fwalk() with os.scandir().

The effect of using os.scandir() in os.fwalk():

$ ./python -m timeit -n1 -r5 -s 'import os' -- 'list(os.walk("/usr/lib"))'
1 loop, best of 5: 934 msec per loop

$ ./python -m timeit -n1 -r5 -s 'import os' -- 'list(os.walk("/usr/lib", topdown=False))'
1 loop, best of 5: 718 msec per loop

$ ./python -m timeit -n1 -r5 -s 'import os' -- 'list(os.fwalk("/usr/lib"))'
Unpatched: 1 loops, best of 5: 1.78 sec per loop
Patched:   1 loop, best of 5: 934 msec per loop

$ ./python -m timeit -n1 -r5 -s 'import os' -- 'list(os.fwalk("/usr/lib", topdown=False))'
Unpatched: 1 loops, best of 5: 1.76 sec per loop
Patched:   1 loop, best of 5: 947 msec per loop