Convert :c:func:`Py_REFCNT` and :c:func:`Py_SIZE` macros to static inline
functions. They cannot be used as l-value anymore: use
:c:func:`Py_SET_REFCNT` and :c:func:`Py_SET_SIZE` to set an object reference
count and size. This change is backward incompatible on purpose, to prepare
the C API for an opaque :c:type:`PyObject` structure.
