GH-101291: Rearrange the size bits in PyLongObject (GH-102464)

* Eliminate all remaining uses of Py_SIZE and Py_SET_SIZE on PyLongObject, adding asserts.

* Change layout of size/sign bits in longobject to support future addition of immortal ints and tagged medium ints.

* Add functions to hide some internals of long object, and for setting sign and digit count.

* Replace uses of IS_MEDIUM_VALUE macro with _PyLong_IsCompact().

Modules/_tkinter.c

@@ -32,6 +32,8 @@ Copyright (C) 1994 Steen Lumholt.
 #  include "pycore_fileutils.h"   // _Py_stat()
 #endif
 
+#include "pycore_long.h"
+
 #ifdef MS_WINDOWS
 #include <windows.h>
 #endif
@@ -886,7 +888,8 @@ asBignumObj(PyObject *value)
     const char *hexchars;
     mp_int bigValue;
 
-    neg = Py_SIZE(value) < 0;
+    assert(PyLong_Check(value));
+    neg = _PyLong_IsNegative((PyLongObject *)value);
     hexstr = _PyLong_Format(value, 16);
     if (hexstr == NULL)
         return NULL;
@@ -1950,7 +1953,7 @@ _tkinter_tkapp_getboolean(TkappObject *self, PyObject *arg)
     int v;
 
     if (PyLong_Check(arg)) { /* int or bool */
-        return PyBool_FromLong(Py_SIZE(arg) != 0);
+        return PyBool_FromLong(!_PyLong_IsZero((PyLongObject *)arg));
     }
 
     if (PyTclObject_Check(arg)) {