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/Objects/structseq.c b/Objects/structseq.c
index 56e885c..091370c 100644
--- a/Objects/structseq.c
+++ b/Objects/structseq.c
@@ -270,7 +270,7 @@
 			Py_DECREF(tup);
 			return NULL;
 		}
-		crepr = PyUnicode_AsString(repr);
+		crepr = _PyUnicode_AsString(repr);
 		if (crepr == NULL) {
 			Py_DECREF(tup);
 			Py_DECREF(repr);