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_validation.c b/tests/utests/data/test_validation.c
index d4f2b47..5b6953d 100644
--- a/tests/utests/data/test_validation.c
+++ b/tests/utests/data/test_validation.c
@@ -492,8 +492,8 @@
     data = "<cont xmlns=\"urn:tests:a\"><a>val</a><b>val_b</b></cont><c xmlns=\"urn:tests:a\">val_c</c>";
     assert_int_equal(LY_SUCCESS, lyd_parse_data_mem(ctx, data, LYD_XML, 0, LYD_VALIDATE_PRESENT, &tree));
     assert_non_null(tree);
-    assert_string_equal("a", lyd_node_children(tree, 0)->schema->name);
-    assert_int_equal(LYD_WHEN_TRUE, lyd_node_children(tree, 0)->flags);
+    assert_string_equal("a", lyd_child(tree)->schema->name);
+    assert_int_equal(LYD_WHEN_TRUE, lyd_child(tree)->flags);
     assert_string_equal("c", tree->next->schema->name);
     assert_int_equal(LYD_WHEN_TRUE, tree->next->flags);
     lyd_free_all(tree);