add CCRef.swap

src/core/CCRef.ml

@@ -32,6 +32,11 @@ let compare f r1 r2 = f !r1 !r2
 
 let equal f r1 r2 = f !r1 !r2
 
+let swap a b =
+  let x = !a in
+  a := !b;
+  b := x
+
 let to_list r = [!r]
 let to_seq r yield = yield !r
 

src/core/CCRef.mli

@@ -31,6 +31,10 @@ val get_then_incr : int t -> int
 (** [get_then_incr r] increments [r] and returns its old value, think [r++]
     @since 0.17 *)
 
+val swap : 'a t -> 'a t -> unit
+(** Swap values.
+    @since NEXT_RELEASE *)
+
 val compare : 'a ord -> 'a t ord
 
 val equal : 'a eq -> 'a t eq