Add antiquotation for structure.

ppx_metaquot.ml

@@ -19,6 +19,7 @@
      [%e ...] where ... is an expression of type Parsetree.expression
      [%t ...] where ... is an expression of type Parsetree.core_type
      [%p ...] where ... is an expression of type Parsetree.pattern
+     [%%s ...] where ... is an expression of type Parsetree.structure
 
 
    All locations generated by the meta quotation are by default set
@@ -67,6 +68,10 @@ module Main : sig end = struct
     | Ldot(m, _) -> String.concat "." (Longident.flatten m) ^ "." ^ s
     | _ -> s
 
+  let append ?loc ?attrs e e' =
+    let fn = Location.mknoloc (Longident.(Ldot (Lident "List", "append"))) in
+    Exp.apply ?loc ?attrs (Exp.ident fn) [Nolabel, e; Nolabel, e']
+
   class exp_builder =
     object
       method record ty x = record (List.map (fun (l, e) -> prefix ty l, e) x)
@@ -135,6 +140,15 @@ module Main : sig end = struct
         | {ppat_desc=Ppat_extension({txt="p";loc}, e); _} -> map (get_exp loc e)
         | x -> super # lift_Parsetree_pattern x
 
+      method! lift_Parsetree_structure str =
+        List.fold_right
+          (function
+           | {pstr_desc=Pstr_extension(({txt="s";loc}, e), _); _} ->
+               append (get_exp loc e)
+           | x ->
+               cons (super # lift_Parsetree_structure_item x))
+          str (nil ())
+
       method! lift_Parsetree_core_type = function
         | {ptyp_desc=Ptyp_extension({txt="t";loc}, e); _} -> map (get_exp loc e)
         | x -> super # lift_Parsetree_core_type x