fn bitslice_swaps<W: Word>(t: &mut [W; 8])Expand description
Width-generic delta-swap pipeline shared by bitslice and inv_bitslice
across every Word impl. The same three-pass sequence inverts itself, so
bitslice and inv_bitslice invoke it identically.
The diagrams below describe the u32 case (8-bit rows); for u64 each
bit position widens by one, but the swap structure is unchanged.