feat(CCRAL): add get_and_remove_exn operation

It seems like a waste to repeat the search for an index to both get
the element at that index and then to remove it. The added
`get_and_remove_exn` operation performs a `remove` but returns the
found element rather than forgetting it.

src/data/CCRAL.ml

@@ -125,6 +125,17 @@ let remove l i = _remove [] l i
   [1;2;4] (to_list @@ remove (of_list [1;2;3;4]) 2)
 *)
 
+let rec _get_and_remove_exn found prefix l i =
+  let x, l' = front_exn l in
+  if i=0
+  then (found := Some x; List.fold_left (fun l x -> cons x l) l' prefix)
+  else _get_and_remove_exn found (x::prefix) l' (i-1)
+
+let get_and_remove_exn l i =
+  let found = ref None in
+  let l' = _get_and_remove_exn found [] l i in
+  (Option.get !found, l')
+
 let rec _map_tree f t = match t with
   | Leaf x -> Leaf (f x)
   | Node (x, l, r) -> Node (f x, _map_tree f l, _map_tree f r)

src/data/CCRAL.mli

@@ -65,7 +65,11 @@ val set : 'a t -> int -> 'a -> 'a t
     @raise Invalid_argument if the list has less than [i+1] elements. *)
 
 val remove : 'a t -> int -> 'a t
-(** [remove l i] removes the [i]-th element of [v].
+(** [remove l i] removes the [i]-th element of [l].
+    @raise Invalid_argument if the list has less than [i+1] elements. *)
+
+val get_and_remove_exn : 'a t -> int -> 'a * 'a t
+(** [get_and_remove_exn l i] accesses and removes the [i]-th element of [l].
     @raise Invalid_argument if the list has less than [i+1] elements. *)
 
 val append : 'a t -> 'a t -> 'a t