Implement kat::linear_grid::collaborative::block::at_warp_stride()

[eyalroz]

We have grid-scope action in two forms - at grid stride and at block stride. The block stride action means each block acts on consecutive data. At block-scope - we only have the first kind of action, where the stride involves all collaborating elements. We don't have the second kind of action, but with warps, which may be told how many consecutive warp's worth of positions to act on.