bpo-39573: Convert Py_REFCNT and Py_SIZE to functions (GH-20429)

Convert Py_REFCNT() and Py_SIZE() macros to static inline functions.
They cannot be used as l-value anymore: use Py_SET_REFCNT() and
Py_SET_SIZE() to set an object reference count and size.

Replace &Py_SIZE(self) with &((PyVarObject*)self)->ob_size
in arraymodule.c.

This change is backward incompatible on purpose, to prepare the C API
for an opaque PyObject structure.

Objects/tupleobject.c

@@ -82,7 +82,7 @@ tuple_alloc(Py_ssize_t size)
         numfree[size]--;
         /* Inline PyObject_InitVar */
 #ifdef Py_TRACE_REFS
-        Py_SIZE(op) = size;
+        Py_SET_SIZE(op, size);
         Py_SET_TYPE(op, &PyTuple_Type);
 #endif
         _Py_NewReference((PyObject *)op);