add (Try)GetNext and (Try)GetPrev to IndexList or ChanableIndexList

For some use cases when manipulating a ChangeableIndexList, it would be convenient to explicitly query the Next and Prev element.

My uses case are:

Recording a undo/redo action of adding/deleting items with an API that only allows Add(item : Item, before : Item) and Remove(item : Item) by storing (item, before)
Selecting next/prev by arrow keys from a state like:
```
clist<Item> : items
clist<Item> : selection
```

Currently there are only the most generic methods IndexList.TryFind(value : 'a) and IndexList.Neighbours(index : Index) and need to be used like this:

let item = 5
let list = clist [1; 5; 10]

let prev = 
    match list.Value.TryFind(item) with 
    | Some ind ->
        let (prev, _, _) = list.Value.Neighbours ind
        prev
    | None -> None

All possible combinations that query by value and by index would be: IndexList.TryGetNext(index : Index) : Index * a' option IndexList.TryGetNext(value : a') : Index * a' option IndexList.TryGetNextValue(value : a') : a' option

Or at the changeable collection itself: ChangeableIndexList.TryGetNext(index : Index) : Index * a' option ChangeableIndexList.TryGetNext(value : a') : Index * a' option ChangeableIndexList.TryGetNextValue(value : a') : a' option

The highlighted one would the perfect solution for my use cases and was previously available on ChangeableOrderedSet, but I would not want to conclude on what the most general ones are based on them.

The ChangeableIndexList could also directly provide more forwarded methods of the IndexList like TryFind.

fsprojects / FSharp.Data.Adaptive

add (Try)GetNext and (Try)GetPrev to IndexList or ChanableIndexList #63