I committed a change to my fork:

This moves direct stack manipulation out of genobject.c into ceval.c and allows me to work on a non-stack way to deal with these tasks (note all the calls to Py_FatalError in the CO_REGISTER branches). I am specifically not holding this up as a proposal for how to do this (I am largely ignorant of many of the internal or CPython-specific aspects of the C API). Still, the tests pass and I can start to address those fatal errors.

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