Make the return of ``IsStartOfNewCallStack`` to be a namedtuple, something like:
('StartOfCallStack', ['priority', 'format_type', 'language_type',
'metadata'])

whenever is_new_callstack is False all the other parts are None,
we can enforce that invariant by returning None whenever it's not a new
callstack. Thus, when it is a new callstack we need only return the
namedtuple, since the is_new_callstack bit is redundant.