bpo-45947: Place dict and values pointer at fixed (negative) offset just before GC header. (GH-29879)

* Place __dict__ immediately before GC header for plain Python objects.

* Fix up lazy dict creation logic to use managed dict pointers.

* Manage values pointer, placing them directly before managed dict pointers.

* Convert hint-based load/store attr specialization target managed dict classes.

* Specialize LOAD_METHOD for managed dict objects.

* Remove unsafe _PyObject_GC_Calloc function.

* Remove unsafe _PyObject_GC_Malloc() function.

* Add comment explaning use of Py_TPFLAGS_MANAGED_DICT.

Python/sysmodule.c

@@ -1681,10 +1681,7 @@ _PySys_GetSizeOf(PyObject *o)
         return (size_t)-1;
     }
 
-    /* add gc_head size */
-    if (_PyObject_IS_GC(o))
-        return ((size_t)size) + sizeof(PyGC_Head);
-    return (size_t)size;
+    return (size_t)size + _PyType_PreHeaderSize(Py_TYPE(o));
 }
 
 static PyObject *