Rollup merge of #81127 - hanmertens:binary_heap_sift_down_perf, r=dtolnay

Improve sift_down performance in BinaryHeap

Replacing `child < end - 1` with `child <= end.saturating_sub(2)` in `BinaryHeap::sift_down_range` (surprisingly) results in a significant speedup of `BinaryHeap::into_sorted_vec`. The same substitution can be done for `BinaryHeap::sift_down_to_bottom`, which causes a slight but probably statistically insignificant speedup for `BinaryHeap::pop`. It's interesting that benchmarks aside from `bench_into_sorted_vec` are barely affected, even those that do use `sift_down_*` methods internally.

| Benchmark                | Before (ns/iter) | After (ns/iter) | Speedup |
|--------------------------|------------------|-----------------|---------|
| bench_find_smallest_1000<sup>1</sup> | 392,617          | 385,200         |    1.02 |
| bench_from_vec<sup>1</sup>           | 506,016          | 504,444         |    1.00 |
| bench_into_sorted_vec<sup>1</sup>    | 476,869          | 384,458         |    1.24 |
| bench_peek_mut_deref_mut<sup>3</sup> | 518,753          | 519,792         |    1.00 |
| bench_pop<sup>2</sup>                | 446,718          | 444,409         |    1.01 |
| bench_push<sup>3</sup>               | 772,481          | 770,208         |    1.00 |

<sup>1</sup>: internally calls `sift_down_range`
<sup>2</sup>: internally calls `sift_down_to_bottom`
<sup>3</sup>: should not be affected

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