Implementing the new C API in CPython

There are approximately 44 thousand occurences of PyObject * in the CPython codebase. That is a lot to change.

However, we don't need to change all the code that uses PyObject * to PyRef overnight. In fact until we change the implementation of PyRef to tagged pointers, or similar, we don't need to change much at all.

Starting with the simplest representation of PyRef as

typedef struct {
    PyObject *ptr;
} PyRef;

Most uses of PyObject * are one of a few simple operations, such as Py_INCREF(), Py_DECREF() and Py_TYPE().

Changing Py_TYPE() is easy enough. Py_TYPE(obj) becomes PyAPI_Object_GetClass(obj). As long as PYAPI_NO_ABI is defined, it remains efficient.

As a stopgap, we can add PyRef_INCREF(), PyRef_DECREF(). Ultimately they will have to go, but we can leave them in for a few years.

Most other uses of PyObject * involving passing it to another C API function, or casting to a more specific class. With the possible future exception of int and float, we can continue to cast. (PyXXXObject *)obj becomes PyApi_XXX_Unsafecast(obj). Converting to safe casts can be done later.

Adding two inline functions should help as well.

PyRef TO_REF(PyObject *);
PyObject *TO_PTR(PyRef ref);