Hashfs

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- Fix out-of-memory issue when computing file ID hashes of large files.

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- Replace usage of ``distutils.dir_util.mkpath`` with ``os.path.makedirs``.

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- Use ``shutil.move`` instead of ``shutil.copy`` to move temporary file created during ``put`` operation to ``HashFS`` directory.

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- Add faster ``scandir`` package for iterating over files/folders when platform is Python < 3.5. Scandir implementation was added to ``os`` module starting with Python 3.5.

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- Rename private method ``HashFS.copy`` to ``HashFS._copy``.
- Add ``is_duplicate`` attribute to ``HashAddress``.
- Make ``HashFS.put()`` return ``HashAddress`` with ``is_duplicate=True`` when file with same hash already exists on disk.

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- Add ``HashFS.size()`` method that returns the size of all files in bytes.
- Add ``HashFS.count()``/``HashFS.__len__()`` methods that return the count of all files.
- Add ``HashFS.__iter__()`` method to support iteration. Proxies to ``HashFS.files()``.
- Add ``HashFS.__contains__()`` method to support ``in`` operator. Proxies to ``HashFS.exists()``.
- Don't create the root directory (if it doesn't exist) until at least one file has been added.
- Fix ``HashFS.repair()`` not using ``extensions`` argument properly.