bpo-39542: Make PyObject_INIT() opaque in limited C API (GH-18363)

In the limited C API, PyObject_INIT() and PyObject_INIT_VAR() are now
defined as aliases to PyObject_Init() and PyObject_InitVar() to make
their implementation opaque. It avoids to leak implementation details
in the limited C API.

Exclude the following functions from the limited C API, move them
from object.h to cpython/object.h:

* _Py_NewReference()
* _Py_ForgetReference()
* _PyTraceMalloc_NewReference()
* _Py_GetRefTotal()