with_custodian_and_ward

It seems there is some demand for a with_custodian_and_ward "call policy" like what is implemented by boost::python:

http://www.boost.org/libs/python/doc/tutorial/doc/html/python/functions.html#python.call_policies
http://www.boost.org/libs/python/doc/v2/reference.html#models_of_call_policies

Possible design:
  The "custodian" object needs to have GC support enabled. The instance structure receives a PyList object that keeps track of wards. This list is deallocated when the custodian is freed, thereby freeing the wards. The list is Py_VISIT'ed in the tp_traverse method, to ensure reference cycles are properly collected. 'ward' Parameter's or ReturnValue's are modified to generate code that appends themselves to the list stored in the custodian.