!122 (merged) introduced a draw_unique_indices function which uses a HashSet to accumulate unique indices in the range 0..<len.
however, a HashSet does not preserve the order of insertion when iterating over the elements of the set... which results in apparent randomness, even though the RNG seed is the same
this MR switches back to using shuffle which used to work, even though a bit less performant
it's basically a revert of !122 (merged), while keeping the refactoring into random.rs.
measuring the performance
i did run the same timing experiment from !122 (merged) but with main on bb55005 and the MR on fix-shuffle
