Draft: BitFieldVec instead of Vec<u64> for path_stack

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added 12 commits

• 98469db8...235f3073 - 11 commits from branch master
• 6014d650 - [testing] BitFieldVec instead of Vec<u64> for path_stack

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this divides the side of the vector by two, but that's at most a couple gigabytes when working with 100 threads, so probably not worth the extra complexity.

Plus, filename ids currently fit on exactly 32 bits, but in a future graph they'll probably need 33 bits, and I'm guessing the mis-alignment might have a performance impact.

closed

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