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_parser_json.c b/tests/utests/data/test_parser_json.c
index 9b79a7b..34101be 100644
--- a/tests/utests/data/test_parser_json.c
+++ b/tests/utests/data/test_parser_json.c
@@ -647,15 +647,15 @@
     /* TODO support generic attributes in JSON ?
     assert_non_null(((struct lyd_node_opaq *)tree)->attr);
     */
-    node = lyd_node_children(tree, 0);
+    node = lyd_child(tree);
     assert_string_equal(node->schema->name, "edit-config");
-    node = lyd_node_children(node, 0)->next;
+    node = lyd_child(node)->next;
     assert_string_equal(node->schema->name, "config");
 
     node = ((struct lyd_node_any *)node)->value.tree;
     assert_non_null(node->schema);
     assert_string_equal(node->schema->name, "cp");
-    node = lyd_node_children(node, 0);
+    node = lyd_child(node);
     /* z has no value */
     assert_null(node->schema);
     assert_string_equal(((struct lyd_node_opaq *)node)->name, "z");
@@ -703,7 +703,7 @@
     assert_null(tree->schema);
     assert_string_equal(((struct lyd_node_opaq *)tree)->name, "rpc");
     assert_null(((struct lyd_node_opaq *)tree)->attr);
-    node = lyd_node_children(tree, 0);
+    node = lyd_child(tree);
     assert_null(node->schema);
     assert_string_equal(((struct lyd_node_opaq *)node)->name, "action");
     assert_null(((struct lyd_node_opaq *)node)->attr);
@@ -747,7 +747,7 @@
     assert_null(tree->schema);
     assert_string_equal(((struct lyd_node_opaq *)tree)->name, "notification");
     assert_null(((struct lyd_node_opaq *)tree)->attr);
-    node = lyd_node_children(tree, 0);
+    node = lyd_child(tree);
     assert_null(node->schema);
     assert_string_equal(((struct lyd_node_opaq *)node)->name, "eventTime");
     assert_string_equal(((struct lyd_node_opaq *)node)->value, "2037-07-08T00:01:00Z");
@@ -813,7 +813,7 @@
 
     assert_non_null(op);
     assert_string_equal(op->schema->name, "act");
-    node = lyd_node_children(op, 0);
+    node = lyd_child(op);
     assert_non_null(node->schema);
     assert_string_equal(node->schema->name, "al");
     assert_true(node->schema->flags & LYS_CONFIG_R);
@@ -821,15 +821,15 @@
     assert_non_null(tree);
     assert_null(tree->schema);
     assert_string_equal(((struct lyd_node_opaq *)tree)->name, "rpc-reply");
-    node = lyd_node_children(tree, 0);
+    node = lyd_child(tree);
     assert_non_null(node->schema);
     assert_string_equal(node->schema->name, "c");
 
     /* TODO print only rpc-reply node and then output subtree */
-    lyd_print_tree(out, lyd_node_children(op, 0), LYD_JSON, LYD_PRINT_SHRINK);
+    lyd_print_tree(out, lyd_child(op), LYD_JSON, LYD_PRINT_SHRINK);
     assert_string_equal(str, "{\"a:al\":25}");
     ly_out_reset(out);
-    lyd_print_tree(out, lyd_node_children(tree, 0), LYD_JSON, LYD_PRINT_SHRINK);
+    lyd_print_tree(out, lyd_child(tree), LYD_JSON, LYD_PRINT_SHRINK);
     assert_string_equal(str, "{\"a:c\":{\"act\":{\"al\":25}}}");
     ly_out_reset(out);
     lyd_free_all(tree);