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/test_xpath.c b/tests/utests/test_xpath.c
index ab4d2c1..57acd0e 100644
--- a/tests/utests/test_xpath.c
+++ b/tests/utests/test_xpath.c
@@ -233,7 +233,7 @@
 
     node = set->objs[0];
     assert_string_equal(node->schema->name, "l1");
-    node = lyd_node_children(node, 0);
+    node = lyd_child(node);
     assert_string_equal(node->schema->name, "a");
     assert_string_equal(LYD_CANON_VALUE(node), "a3");
 
@@ -245,7 +245,7 @@
 
     node = set->objs[0];
     assert_string_equal(node->schema->name, "ll");
-    node = lyd_node_children(node, 0);
+    node = lyd_child(node);
     assert_string_equal(node->schema->name, "a");
     assert_string_equal(LYD_CANON_VALUE(node), "val_b");
     node = node->next;