Fix issue with Binding_tree.add_all

See thizanne/ocaml-hamt#41

src/syntax/common/binding_tree.ml

@@ -51,7 +51,19 @@ let add qualified_identifier entry ?(subtree = empty) tree =
       add_nested namespaces identifier { entry; subtree } tree)
 
 let add_all t1 t2 =
-  Identifier.Hamt.union (fun _identifier _e1 e2 -> Option.some e2) t1 t2
+  (*=
+    It may be more efficient to use the following:
+    {[
+      Identifier.Hamt.union (fun _identifier _e1 e2 -> Option.some e2) t1 t2
+    ]}
+
+    However, there is a bug in [Identifier.Hamt.union]:
+    https://github.com/thizanne/ocaml-hamt/issues/41
+  *)
+  Identifier.Hamt.fold
+    (fun identifier entry accumulator ->
+      Identifier.Hamt.add identifier entry accumulator)
+    t2 t1
 
 let remove identifier tree =
   match Identifier.Hamt.find_opt identifier tree with