patch 8.2.0935: flattening a list with existing code is slow

Problem:    Flattening a list with existing code is slow.
Solution:   Add flatten(). (Mopp, closes #3676)
diff --git a/src/proto/list.pro b/src/proto/list.pro
index 77183af..bdbf7eb 100644
--- a/src/proto/list.pro
+++ b/src/proto/list.pro
@@ -30,6 +30,7 @@
 int list_append_number(list_T *l, varnumber_T n);
 int list_insert_tv(list_T *l, typval_T *tv, listitem_T *item);
 void list_insert(list_T *l, listitem_T *ni, listitem_T *item);
+void f_flatten(typval_T *argvars, typval_T *rettv);
 int list_extend(list_T *l1, list_T *l2, listitem_T *bef);
 int list_concat(list_T *l1, list_T *l2, typval_T *tv);
 list_T *list_copy(list_T *orig, int deep, int copyID);
@@ -37,7 +38,7 @@
 char_u *list2string(typval_T *tv, int copyID, int restore_copyID);
 int list_join(garray_T *gap, list_T *l, char_u *sep, int echo_style, int restore_copyID, int copyID);
 void f_join(typval_T *argvars, typval_T *rettv);
-int get_list_tv(char_u **arg, typval_T *rettv, int evaluate, int do_error);
+int get_list_tv(char_u **arg, typval_T *rettv, int flags, int do_error);
 int write_list(FILE *fd, list_T *list, int binary);
 void init_static_list(staticList10_T *sl);
 void f_list2str(typval_T *argvars, typval_T *rettv);