so that it can be used in more places.  Looking at pyxl_bench2_10x,
most of the calls to PyType_GenericAlloc are from the dict and list
iterator classes, which don't use DEFAULT_CLASS_SIMPLE (or DEFAULT_CLASS)
since they use the same C++ class with multiple Python classes.  So
by extracting out the core of that function, it's now usable by those
other classes.

Also convert some things to DEFAULT_CLASS_SIMPLE while we're at it.