data tree CHANGE move macros to functions to cleanup API

Remove lyd_node_children() since it is just an old duplication of
LYD_CHILD and LYD_CHILD_NO_KEYS macros. As a consequence, these macros
are converted to the functions together with LYD_PARENT (same names, but
lowercase) since they have similar functionality in the API.
diff --git a/tests/utests/data/test_merge.c b/tests/utests/data/test_merge.c
index 5d3050c..1206994 100644
--- a/tests/utests/data/test_merge.c
+++ b/tests/utests/data/test_merge.c
@@ -574,8 +574,8 @@
     st->source = NULL;
 
     /* c should be replaced and now be default */
-    assert_string_equal(lyd_node_children(st->target, 0)->prev->schema->name, "c");
-    assert_true(lyd_node_children(st->target, 0)->prev->flags & LYD_DEFAULT);
+    assert_string_equal(lyd_child(st->target)->prev->schema->name, "c");
+    assert_true(lyd_child(st->target)->prev->flags & LYD_DEFAULT);
 }
 
 static void
@@ -612,7 +612,7 @@
     assert_int_equal(lyd_merge_siblings(&st->target, st->source, 0), LY_SUCCESS);
 
     /* c should not be replaced, so c remains not default */
-    assert_false(lyd_node_children(st->target, 0)->flags & LYD_DEFAULT);
+    assert_false(lyd_child(st->target)->flags & LYD_DEFAULT);
 }
 
 static void