Avoid allocation in no-op update

src/kcas/kcas.ml

@@ -784,7 +784,10 @@ module Xt = struct
     end
     else
       let current = state_r.after in
-      let state = State { state_r with after = f current } in
+      let after = f current in
+      let state =
+        if current == after then state else State { state_r with after }
+      in
       xt.cass <- Cas { loc; state; lt; gt; awaiters = [] };
       current