Fix ppx mapper and add example.

The arity attribute goes on the `Function$` ast node, not the `Pexp_fun` one.

compiler/ml/ast_mapper_from0.ml

@@ -319,9 +319,7 @@ module E = struct
           | [] -> assert false
         in
         match arg1 with
-        | Some
-            ({pexp_desc = Pexp_fun (l, eo, p, e, _); pexp_attributes = attrs} as
-             e1) ->
+        | Some ({pexp_desc = Pexp_fun (l, eo, p, e, _)} as e1) ->
           let arity = attributes_to_arity attrs in
           {e1 with pexp_desc = Pexp_fun (l, eo, p, e, Some arity)}
         | _ -> exp1)

compiler/ml/ast_mapper_to0.ml

@@ -303,13 +303,8 @@ module E = struct
                 ] );
           ]
         in
-        let e =
-          {
-            e with
-            pexp_attributes = arity_to_attributes arity @ e.pexp_attributes;
-          }
-        in
         Ast_helper0.Exp.construct
+          ~attrs:(arity_to_attributes arity)
           (Location.mkloc (Longident.Lident "Function$") e.pexp_loc)
           (Some e))
     | Pexp_apply (e, l) ->

tests/tools_tests/ppx/TestPpx.res

@@ -29,3 +29,5 @@ module Arity = {
 external useState: (unit => 'state) => string = "useState"
 
 let _ = useState(() => 0)
+
+let fpromise = async (promise, _x) => await promise

tests/tools_tests/src/expected/TestPpx.res.jsout

@@ -35,6 +35,10 @@ let Arity = {
 
 React.useState(() => 0);
 
+async function fpromise(promise, _x) {
+  return await promise;
+}
+
 let a = "A";
 
 let b = "B";
@@ -47,4 +51,5 @@ exports.M = M;
 exports.vv = vv;
 exports.OptionalFields = OptionalFields;
 exports.Arity = Arity;
+exports.fpromise = fpromise;
 /*  Not a pure module */