Speedup heapsort by 1.5x by making it branchless.

`slice::sort_unstable` will fall back to heapsort if it repeatedly fails to find
a good pivot. By making the core child update code branchless it is much faster.
On Zen3 sorting 10k `u64` and forcing the sort to pick heapsort, results in:

455us -> 278us

Author: n3vu0r

src/heap_sort.rs

@@ -20,9 +20,7 @@ where
 			}
 
 			// Choose the greater child.
-			if child + 1 < v.len() && is_less(&v[child], &v[child + 1]) {
-				child += 1;
-			}
+			child += (child + 1 < v.len() && is_less(&v[child], &v[child + 1])) as usize;
 
 			// Stop if the invariant holds at `node`.
 			if !is_less(&v[node], &v[child]) {