Improve BinaryHeap performance
By changing the condition in the loops from `child < end` to `child < end - 1` we're guaranteed that `right = child + 1 < end` and since finding the index of the biggest sibling can be done with an arithmetic operation we can remove a branch from the loop body. The case where there's no right child, i.e. `child == end - 1` is instead handled outside the loop, after it ends; note that if the loops ends early we can use `return` instead of `break` since the check `child == end - 1` will surely fail.
I've also removed a call to `<[T]>::swap` that was hiding a bound check that [wasn't being optimized by LLVM](https://godbolt.org/z/zrhdGM).
A quick benchmarks on my pc shows that the gains are pretty significant:
|name |before ns/iter |after ns/iter |diff ns/iter |diff % |speedup |
|---------------------|----------------|---------------|--------------|----------|--------|
|find_smallest_1000 | 352,565 | 260,098 | -92,467 | -26.23% | x 1.36 |
|from_vec | 676,795 | 473,934 | -202,861 | -29.97% | x 1.43 |
|into_sorted_vec | 469,511 | 304,275 | -165,236 | -35.19% | x 1.54 |
|pop | 483,198 | 373,778 | -109,420 | -22.64% | x 1.29 |
The other 2 benchmarks for `BinaryHeap` (`peek_mut_deref_mut` and `push`) weren't impacted and as such didn't show any significant change.
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