The documentation really should mention the resifting, and why this is useful. It only mentions that the heap may be inconsistent if the `PeekMut` isn't proper

The documentation really should mention the resifting, and why this is useful. It only mentions that the heap may be inconsistent if the `PeekMut` isn't properly dropped, without explaining why. It's basically replacing the former (never stabilized) `push_pop` and `replace`, which Python folks can relate to similar methods in their `heapq`, for instance. With separate `push` and `pop` calls, the heap must be sifted twice, but with `peek_mut` you can update the head in-place and sift only once.