bpo-40941: Unify implicit and explicit state in the frame and generator objects into a single value. (GH-20803)

* Merge gen and frame state variables into one.

* Replace stack pointer with depth in PyFrameObject. Makes code easier to read and saves a word of memory.

Modules/_xxsubinterpretersmodule.c

@@ -1847,7 +1847,7 @@ _is_running(PyInterpreterState *interp)
         return 0;
     }
 
-    int executing = (int)(frame->f_executing);
+    int executing = _PyFrame_IsExecuting(frame);
     Py_DECREF(frame);
 
     return executing;