Use operator new() to get access to tp_alloc in a C++-ish way.

Instead of passing the object's class to the constructor, pass
it to new().  We need to do this since the class gets to pick how
the object gets allocated, including how large it will end up being.

Unfortunately it looks like defining an operator new on a subclass
hides the operator new from a superclass, even if they have different
signatures.  Macros to the rescue.