Address review feedback

1. Validate sizes of output arrays (if provided)

2. Remove n_edges parameter, since it can be inferred
   from the output arrays (if provided)

3. Express the idea that the raw pointers interface
   currently lets users provide either out,
   (out_src and out_dst), or (out, out_src, and out_dst).
   Make users pass in the mdspan in a way that
   prevents run-time errors as early as possible.