DirectoryStore `.keys()` optimisation.

It looks like the keys() method of directory store reimplement a lot of the logic of os.walk() that could be used to increase efficiency.

in particular using os.walk() will directly return directory and files separately and will avoid the two extra expensive call to os.path.isfile(path), and os.path.isdir(path).

forming the keys will be a tiny bit less straightforward than current method as os.walk() returns the full relative path you gave it whether or not you terminate it by a slash, so we might have to be careful with this.

(I also note that this methods does list os specific files, like .DStore.. not sure this is on purpose.

[Carreau]

I do have a prototype which appear to be a bit faster on small zarr on my laptop.

In [31]: %timeit list(sorted(z.store.keys_2()))
621 µs ± 8.48 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)

In [32]: %timeit list(sorted(z.store.keys()))
2.05 ms ± 47.1 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Some tests do seem to rely on keys ordering though…

[Carreau]

Notes, os.walk() may also return path with \ instead of /,

And yes os.scandir is used inside os.walk()