data tree FEATURE lyd_new_path() and lyd_change_term()

With tests included. Also, another missing API
function lyd_new_opaq() added.
diff --git a/src/context.c b/src/context.c
index 52a34a5..9f18510 100644
--- a/src/context.c
+++ b/src/context.c
@@ -771,10 +771,9 @@
         root_bis = 0;
     }
 
-    /* TODO uncomment once lefref validation works
     if (lyd_validate(&root, NULL, LYD_VALOPT_DATA_ONLY)) {
         goto error;
-    }*/
+    }
 
     return root;