at end of collapse, replaced copy with a rename.

`warning_f_` adds the exception to the logged message when `~exn` is passed along:
```ocaml
let section = Lwt_log.Section.main

let main () =
    let exn = Failure "Fail!" in
    Logger.warning_f_ ~exn "Foo"
;;

Lwt_main.run (main ())
```

```
$ ./test_logger.native 
test_logger.native: main: Foo: Failure("Fail!")
```