Add init method to lists

test/addressbook.capnp

@@ -37,3 +37,6 @@ struct AddressBook {
   people @0 :List(Person);
 }
 
+struct NestedList {
+  list @0 :List(List(Int32));
+}

test/test_struct.py

@@ -218,3 +218,16 @@ def test_to_dict_ordered(addressbook):
     else:
         with pytest.raises(Exception):
             person.to_dict(ordered=True)
+
+def test_nested_list(addressbook):
+    struct = addressbook.NestedList.new_message()
+    struct.init('list', 2)
+
+    struct.list.init(0, 1)
+    struct.list.init(1, 2)
+
+    struct.list[0][0] = 1
+    struct.list[1][0] = 2
+    struct.list[1][1] = 3
+
+    assert struct.to_dict()["list"] == [[1], [2,3]]