pyclass: refactor tp_new / tp_dealloc / remove PyCellInner

[davidhewitt]

This PR is a bunch of cleanups to PyCell and PyClassInitializer code for the slot implementations of tp_new, tp_alloc, tp_dealloc and tp_free:

• tp_alloc and tp_free are only implemented for freelist classes, where they provide custom memory allocation / release
• tp_new now does the same thing for all #[pyclass] (call super tp_new and initialize the PyCell contents)
• tp_dealloc does the reverse of tp_new (drop the PyCell contents and then call super tp_dealloc)

This work seems necessary to me as a precursor to #1647, #1637, #1344 and #991.

While doing this I also found a nasty bug that we don't call tp_dealloc of inherited native types, which means that dict subclasses never decrease reference counts of all their contents on drop (for example).

I added a benchmark for class instantiation. Sadly these cleanups don't seem to have meaningfully changed the benchmark. I did notice that we're a lot faster than the numbers reported in #661, so there must have been progress over time.

[davidhewitt]

Oooh nice, a segfault purely on windows 32-bit 👀

[davidhewitt]

Turns out I forgot #[repr(C)] from PartiallyInitializedPyCell 😬

[davidhewitt]

If nobody reviews of raises any concerns by the weekend I plan to merge this.