Implement a `replace_full` method on `IndexSet`

[zakcutner]

Hello! I was wondering whether it would be possible to implement a replace_full method on IndexSet? Similarly to how there are the insert and insert_full methods, I think it might also be useful to also have a replace_full method.

pub fn replace_full(&mut self, value: T) -> (usize, Option<T>)

I believe the semantics would be:

• If the item already exists, replace it with value and return its index (which will remain unchanged) as well as its old value.
• Otherwise, insert the item into the set and return its index as well as None.

[cuviper]

Do you mean on IndexSet? If so, that makes sense to me.

[zakcutner]

Do you mean on IndexSet?

Yes, I do 😅 Sorry, I've updated the issue!

[zakcutner]

I went ahead and created #222 with a possible implementation, I hope that's okay 🙂