[ppx_trace] Refactor: Avoid using Ast_mapper for ppxlib 0.16 compatibility

Summary:
Use `Ppxlib.Ast_traverse.map` instead of
`Ppxlib.Selected_ast.Ast.Ast_mapper` which is included from
`Migrate_parsetree` since `Ppxlib.Selected_ast.Ast` reexports one of
the `Migrate_parsetree.Versions` modules. This change is needed to be
compatible with (ocaml-migrate-parsetree 2.0 and) ppxlib 0.16 since it
no longer re-exports the `Ast_mapper` module from
`Migrate_parsetree`. This ppxlib change is one of the headline
simplification enablers noted in the (announcement of
ocaml-migrate-parsetree
2.0)[https://discuss.ocaml.org/t/ocaml-migrate-parsetree-2-0-0/5991].

Reviewed By: jvillard

Differential Revision: D23636203

fbshipit-source-id: 71e24b46b

sledge/ppx_trace/ppx_trace.ml

@@ -40,7 +40,6 @@
 
 open Ppxlib
 open Ast_builder.Default
-module Ast_mapper = Selected_ast.Ast.Ast_mapper
 
 let debug = ref false
 
@@ -75,11 +74,13 @@ let vb_stack_with, vb_stack_top =
   (with_, top)
 
 let mapper =
-  let value_binding (m : Ast_mapper.mapper) vb =
-    vb_stack_with vb.pvb_pat ~f:(fun () ->
-        Ast_mapper.default_mapper.value_binding m vb )
-  in
-  let expr (m : Ast_mapper.mapper) exp =
+  object
+    inherit Ast_traverse.map as super
+
+    method! value_binding vb =
+      vb_stack_with vb.pvb_pat ~f:(fun () -> super#value_binding vb)
+
+    method! expression exp =
       let append_here_args args =
         let mod_name = evar ~loc:Location.none "Stdlib.__MODULE__" in
         let fun_name =
@@ -120,11 +121,10 @@ let mapper =
             pexp_apply ~loc:exp.pexp_loc
               (evar ~loc:retn_loc "Trace.retn")
               (append_here_args [(Nolabel, retn_fun)])
-    | _ -> Ast_mapper.default_mapper.expr m exp
-  in
-  {Ast_mapper.default_mapper with expr; value_binding}
+      | _ -> super#expression exp
+  end
 
-let impl = Selected_ast.Ast.map_structure mapper
+let impl = mapper#structure
 
 ;;
 Driver.register_transformation "trace" ~impl