A modest speedup of object deallocation. call_finalizer() did rather

a lot of work: it had to save and restore the current exception around
a call to lookup_maybe(), because that could fail in rare cases, and
most objects don't have a __del__ method, so the whole exercise was
usually a waste of time.  Changed this to cache the __del__ method in
the type object just like all other special methods, in a new slot
tp_del.  So now subtype_dealloc() can test whether tp_del is NULL and
skip the whole exercise if it is.  The new slot doesn't need a new
flag bit: subtype_dealloc() is only called if the type was dynamically
allocated by type_new(), so it's guaranteed to have all current slots.
Types defined in C cannot fill in tp_del with a function of their own,
so there's no corresponding "wrapper".  (That functionality is already
available through tp_dealloc.)