Format a #TpHandleSet for debug output.
Returns a dictionary mapping each handle in self
to the corresponding
identifier, as if retrieved by calling tp_handle_inspect() on each handle.
The type of the returned value is described as
Handle_Identifier_Map
in the Telepathy specification.
A set of handles. This is similar to a #TpIntset (and implemented using one), but adding a handle to the set also references it.