Enhanced byte-chunking of directories

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This issue describes formalised techniques for hashing directories to consider nipype/pydra#473. When implementing, could combine with this gist for multithreading, https://gist.github.com/satra/0e02cd7554672120cf8073df3986f302

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@jamesdidathing not urgent, but if you have some spare cycles at some point this might add a nice performance boost for the workflows engine