map needs labelled arg, bind doesn't?

Just tested ppx_let with BuckleScript, and while `let%bind` was working fine for me, I was struggling getting `let%map` to work.

It seems that

```ocaml
let p = Js.Promise.resolve 1;;
let p1 = let%bind x = p in Js.Promise.resolve (x + 1);;
let p2 = let%map x = p in x + 1;;
```

gets transformed by the ppx to

```ocaml
let p = Js.Promise.resolve 1
let p1 = Let_syntax.bind p (fun x -> Js.Promise.resolve (x + 1))
let p2 = Let_syntax.map p ~f:(fun x -> x + 1)
```

which means that map needs a labelled arg `~f`, but bind doesn't. So the definition of map should actually be

```ocaml
let map promise ~f =
Js.Promise.then_ (fun v -> Js.Promise.resolve (f v)) promise
```

What is super weird is that according to the ppx_let docs, *both* bind and map should require a labelled arg for `f`.