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/bytesobject.c b/Objects/bytesobject.c
index eeae0ff..7c1469c 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -3230,7 +3230,7 @@
 	if (!result)
 		return NULL;
 
-	buf = PyUnicode_AsString(result);
+	buf = _PyUnicode_AsString(result);
 	if (!buf) {
 		Py_DECREF(result);
 		return NULL;