Elide bounds check in diffxi

array_windows.skip did not elide bounds checks as it should. If
the slice is instead offset by the skipped amount, we have the
same behavour, but aids the compiler enough.
The two changed lines allows SIMD optimisations, giving an
impressive reduction in instructions by two thirds in the
benchmark.

sbp/src/operators/algos.rs

@@ -142,9 +142,8 @@ pub(crate) fn diff_op_1d_slice<const M: usize, const N: usize, const D: usize>(
     // based on the block size
     let window_elems_to_skip = M - ((D - 1) / 2);
 
-    for (window, f) in prev
+    for (window, f) in prev[window_elems_to_skip..]
         .array_windows::<D>()
-        .skip(window_elems_to_skip)
         .zip(fut.array_chunks_mut::<1>())
     {
         let fut = ColVector::<_, 1>::map_to_col_mut(f);