Rename PyUnicode_AsString -> _PyUnicode_AsString and

PyUnicode_AsStringAndSize -> _PyUnicode_AsStringAndSize to mark
them for interpreter internal use only.

We'll have to rework these APIs or create new ones for the
purpose of accessing the UTF-8 representation of Unicode objects
for 3.1.

Modules/operator.c

@@ -449,7 +449,7 @@ dotted_getattr(PyObject *obj, PyObject *attr)
 		return NULL;
 	}
 
-	s = PyUnicode_AsString(attr);
+	s = _PyUnicode_AsString(attr);
 	Py_INCREF(obj);
 	for (;;) {
 		PyObject *newobj, *str;