Gs-wrap

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* Changed argument in ``google.cloud.storage.client.Client.get_bucket``
since interface broke with google-cloud-storage 1.16.0

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* ``crc32c`` and ``md5`` in ``Stat`` are decoded into actual bytes.
Google client sends crc32s and md5 of the objects base-64 encoded which
is confusing for the end users.

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* Added mypy --strict flag in precommit.py and fixed corresponding errors
* Fixed mkdir race condition when downloading
* Updated docstrings with examples
* Re-structured multi-threading execution by adding nested functions and titles
for readability
* Excluded benchmark module from distribution
* Updated dependencies and re-formatted code for new yapf version

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* Fixed downloading a file into a directory bug and added testcase
* Fixed long_ls return type

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* Rephrase PyPi description in meta information about gs-wrap package

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* Initial version