rcb: use crate::partial_cmp for select_nth

Since this makes half as many float comparisons and select_nth_unstable
ends up only matching for Ordering::Less anyway:
https://github.com/rust-lang/rust/blob/cb0584f86b8cfa952dffad55f7d83bd90765120f/library/core/src/slice/mod.rs#L2719

Author: hhirtz

src/algorithms/recursive_bisection.rs

@@ -135,7 +135,7 @@ fn rcb_recurse<const D: usize, W>(
 
         let (left, _, _right_minus_one) = items
             .select_nth_unstable_by(split_idx, |item1, item2| {
-                f64::partial_cmp(&item1.point[coord], &item2.point[coord]).unwrap()
+                crate::partial_cmp(&item1.point[coord], &item2.point[coord])
             });
         let left_len = left.len();
         items.split_at_mut(left_len)