&mut self APIs for skiplist

I'm thinking it would be useful if we could have &mut self APIs like get_mut, insert_mut, remove_mut and iter_mut for skiplist.

In recent months I was working on a relatively similar scenario for queues in PR #1191 and I think same benefits in terms of performance and even more features like giving mutable access to a value with get_mut and etc can be obtained here for skiplist.

If it makes any sense, I can take time working on it.

thanks