[lang] Add parallel scan prefix sum utility

[qiao-bo]

This is an single-buffer implementation of parallel scan (prefix sum).

Reference: https://developer.download.nvidia.com/compute/cuda/1.1-Beta/x86_website/projects/scan/doc/sc https://github.com/NVIDIA/cuda-samples/blob/master/Samples/2_Concepts_and_Techniques/shfl_sc

This will be useful for certain Taichi implementations such as PBD.

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[qiao-bo]

/rebase