Consider adding `BinaryHeap::peek_mut` like API for `Vec` and `VecDeque`

There are times when I want to remove an element from a collection only if it meets some conditions. For example, I want to remove the last item from a Vec<i32> if it exists and is less than 10. Currently I can think of two ways to do this:

fn pop_if_less_than_10(values: &mut Vec<i32>) -> Option<i32> {
    values.pop().and_then(|value| {
        if value < 10 {
            Some(value)
        } else {
            values.push(value);

            None
        }
    })
}

Or I can write:

fn pop_if_less_than_10(values: &mut Vec<i32>) -> Option<i32> {
    if values.last().map_or(false, |value| *value < 10) {
        Some(values.pop().unwrap())
    } else {
        None
    }
}

The first one may pop and push the same element which is the same as doing nothing, and the second one has a infallible unwrap. I think the idea of BinaryHeap::peek_mut may be applied here:

fn pop_if_less_than_10(values: &mut Vec<i32>) -> Option<i32> {
    values.peek_last_mut().and_then(|last| {
        if *last < 10 {
            Some(PeekLastMut::pop(last))
        } else {
            None
        }
    })
}

Where peek_last_mut is an imaginary method like BinaryHeap::peek_mut and PeekLastMut is an imaginary type is like std::collections::binary_heap::PeekMut.

The entry methods from HashMap and BTreeMap are similar, but we have no way of doing this in Vec and VecDeque. It would be nice to have something like this.

Start by comparing the proposed Vec and VecDeque operation with BinaryHeap::peek_mut and the existing collection APIs mentioned in the issue. Work out the intended API and ownership or drop semantics, then define what a complete proposal would need to specify; the issue does not name files or tests.