bpo-30860: Consolidate stateful runtime globals. (#2594)

* group the (stateful) runtime globals into various topical structs
* consolidate the topical structs under a single top-level _PyRuntimeState struct
* add a check-c-globals.py script that helps identify runtime globals

Other globals are excluded (see globals.txt and check-c-globals.py).

Objects/setobject.c

@@ -1115,6 +1115,7 @@ frozenset_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     }
     /* The empty frozenset is a singleton */
     if (emptyfrozenset == NULL)
+        /* There is a possible (relatively harmless) race here. */
         emptyfrozenset = make_new_set(type, NULL);
     Py_XINCREF(emptyfrozenset);
     return emptyfrozenset;