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/src/parser_xml.c b/src/parser_xml.c
index 43f6925..914b643 100644
--- a/src/parser_xml.c
+++ b/src/parser_xml.c
@@ -1022,7 +1022,7 @@
     if (op_p) {
         *op_p = rep_op;
     }
-    for (tree = rep_op; tree->parent; tree = LYD_PARENT(tree)) {}
+    for (tree = rep_op; tree->parent; tree = lyd_parent(tree)) {}
     if (rpcr_e) {
         /* connect to the operation */
         lyd_insert_node(rpcr_e, NULL, tree);