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).

diff --git a/Python/thread.c b/Python/thread.c
index 4d2f2c3..6fd594f 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -76,11 +76,6 @@
     PyThread__init_thread();
 }
 
-/* Support for runtime thread stack size tuning.
-   A value of 0 means using the platform's default stack size
-   or the size specified by the THREAD_STACK_SIZE macro. */
-static size_t _pythread_stacksize = 0;
-
 #if defined(_POSIX_THREADS)
 #   define PYTHREAD_NAME "pthread"
 #   include "thread_pthread.h"
@@ -96,7 +91,7 @@
 size_t
 PyThread_get_stacksize(void)
 {
-    return _pythread_stacksize;
+    return PyThreadState_GET()->interp->pythread_stacksize;
 }
 
 /* Only platforms defining a THREAD_SET_STACKSIZE() macro