Add tulip.wait_for().

pep-3156.txt

@@ -1329,6 +1329,12 @@ package are provided:
   your ``for`` loop may not make progress (since you are not allowing
   other tasks to run).
 
+- ``tulip.wait_for(f, timeout)``.  This is a convenience to wait for a
+  single coroutine or Future with a timeout.  It is a simple wrapper
+  around ``tulip.wait()`` with a single item in the first argument,
+  returning the result or raising the exception if it is completed
+  within the timeout, raising ``TimeoutError`` otherwise.
+
 Sleeping
 --------