Also, the basic rules of thumb I use for deciding whether or not a setting belongs in CoreConfig:

* does `PyUnicode_New` need this? (If yes, then include it)
* does the importlib bootstrapping need this? (If yes, then include it)

Everything else goes in MainInterpreterConfig as a real Python object.

We may need other structs internally to help manage the way Py_Main populates MainInterpreterConfig, but those should be made a required part of the future public initialization API (although we may decide to expose them as "use them if you want to better emulate CPython's default behaviour" helper APIs).