patch 8.0.0593: duplication of code for adding a list or dict return value

Problem:    Duplication of code for adding a list or dict return value.
Solution:   Add rettv_dict_set() and rettv_list_set(). (Yegappan Lakshmanan)
diff --git a/src/proto/dict.pro b/src/proto/dict.pro
index 61f4dfa..2a76263 100644
--- a/src/proto/dict.pro
+++ b/src/proto/dict.pro
@@ -1,6 +1,7 @@
 /* dict.c */
 dict_T *dict_alloc(void);
 int rettv_dict_alloc(typval_T *rettv);
+void rettv_dict_set(typval_T *rettv, dict_T *d);
 void dict_unref(dict_T *d);
 int dict_free_nonref(int copyID);
 void dict_free_items(int copyID);