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/path.c b/src/path.c
index 03d735a..07cd775 100644
--- a/src/path.c
+++ b/src/path.c
@@ -820,7 +820,7 @@
 
     if (lysc_data_parent(path[0].node)) {
         /* relative path, start from the parent children */
-        start = lyd_node_children(start, 0);
+        start = lyd_child(start);
     } else {
         /* absolute path, start from the first top-level sibling */
         while (start->parent) {
@@ -870,7 +870,7 @@
         prev_node = node;
 
         /* next path segment, if any */
-        start = lyd_node_children(node, 0);
+        start = lyd_child(node);
     }
 
     if (node) {