feat(tree): add rwlock

include/vieter_tree.h

@@ -67,4 +67,22 @@ void vieter_tree_iterator_free(vieter_tree_iterator **ptp);
 vieter_tree_error vieter_tree_iterator_next(void **out,
                                             vieter_tree_iterator *iter);
 
+/*
+ * Acquire a read lock on the tree. Return value is the result of
+ * pthread_rwlock_rdlock.
+ */
+int vieter_tree_rlock(vieter_tree *tree);
+
+/*
+ * Acquire a write lock on the tree. Return value is the result of
+ * pthread_rwlock_wrlock.
+ */
+int vieter_tree_wlock(vieter_tree *tree);
+
+/*
+ * Unlock the lock after having acquired it. Return value is the result of
+ * pthread_rwlock_unlock.
+ */
+int vieter_tree_unlock(vieter_tree *tree);
+
 #endif