[FEA]: Provide `cuda::fns` - Find the position of the n-th set

[fbusato]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this request and that I agree to the Code of Conduct

Area

libcu++

Is your feature request related to a problem? Please describe.

@nanan-nvidia discovered that the CUDA intrinsic __fns performance was a bottleneck for RLE optimization.
He proposed an optimized version based on binary search.

Describe the solution you'd like

Generalization of Find the position of the n-th set provided in https://github.com/NVIDIA/cccl/blob/main/cub/cub/device/dispatch/kernels/kernel_rle_encode_lookahead.cuh#L198 that works with all (unsigned) integral types

Potentially considering further optimizations, e.g. Select the bit position in https://graphics.stanford.edu/~seander/bithacks.html
and CPU instructions x86 BMI2 PEXT or Arm SVE2 BEXT

Describe alternatives you've considered

No response

Additional context

No response