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.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index a1e56be..43be9ed 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -406,7 +406,7 @@
 		if (msg == NULL)
 			return -1;
 		rval = PyErr_WarnEx(PyExc_DeprecationWarning,
-				    PyUnicode_AsString(msg), 2);
+				    _PyUnicode_AsString(msg), 2);
 		Py_DECREF(msg);
 		if (rval == 0)
 			return 0;