mapping object", in the same sense dict.update(x) requires of x (that x
has a keys() method and a getitem).
Questionable:  The other type constructors accept a keyword argument, so I
did that here too (e.g., dictionary(mapping={1:2}) works).  But type_call
doesn't pass the keyword args to the tp_new slot (it passes NULL), it only
passes them to the tp_init slot, so getting at them required adding a
tp_init slot to dicts.  Looks like that makes the normal case (i.e., no
args at all) a little slower (the time it takes to call dict.tp_init and
have it figure out there's nothing to do).