I think I've fixed the memory leaks (plural).

There were also a host of other problems with the _UNPACK opcodes in ceval. Here are the things I remember fixing, although I think I did slightly more:

- Not throwing an error when PyDict_New or PyDict_Update fails.
- Not doing Py_DECREF on stack items being popped.
- Not checking if intersection is non-NULL.
- Not doing Py_DECREF on intersection.

Now the primary problem is giving good errors; I don't know how to make them look like they came from the function call. I'm not sure I want to, either, since these opcodes are used elsewhere.

I do need to check something about this (what requirements are there on how you leave the stack when you "goto error"?), but that's an easy fix if my current guess isn't right.