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/printer_json.c b/src/printer_json.c
index 2a8a876..2cf8cef 100644
--- a/src/printer_json.c
+++ b/src/printer_json.c
@@ -591,7 +591,7 @@
 json_print_inner(struct jsonpr_ctx *ctx, const struct lyd_node *node)
 {
     struct lyd_node *child;
-    struct lyd_node *children = lyd_node_children(node, 0);
+    struct lyd_node *children = lyd_child(node);
     ly_bool has_content = 0;
 
     if (node->meta || children) {
@@ -670,7 +670,7 @@
     }
 
     if (node->schema->nodetype == LYS_LIST) {
-        if (!lyd_node_children(node, 0)) {
+        if (!lyd_child(node)) {
             /* empty, e.g. in case of filter */
             ly_print_(ctx->out, "%s%snull", (ctx->level_printed >= ctx->level) ? "," : "", DO_FORMAT ? " " : "");
             LEVEL_PRINTED;