bpo-45953: Statically allocate the main interpreter (and initial thread state). (gh-29883)

Previously, the main interpreter was allocated on the heap during runtime initialization.  Here we instead embed it into _PyRuntimeState, which means it is statically allocated as part of the _PyRuntime global.  The same goes for the initial thread state (of each interpreter, including the main one).  Consequently there are fewer allocations during runtime/interpreter init, fewer possible failures, and better memory locality.

FYI, this also helps efforts to consolidate globals, which in turns helps work on subinterpreter isolation.

https://bugs.python.org/issue45953

Python/ceval.c

@@ -617,7 +617,7 @@ Py_AddPendingCall(int (*func)(void *), void *arg)
     }
     else {
         /* Last resort: use the main interpreter */
-        interp = _PyRuntime.interpreters.main;
+        interp = _PyInterpreterState_Main();
     }
     return _PyEval_AddPendingCall(interp, func, arg);
 }