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/Python/structmember.c b/Python/structmember.c
index 6c1e932..9f08a6b 100644
--- a/Python/structmember.c
+++ b/Python/structmember.c
@@ -247,7 +247,7 @@
 			PyErr_BadArgument();
 			return -1;
 		}
-		string = PyUnicode_AsStringAndSize(v, &len);
+		string = _PyUnicode_AsStringAndSize(v, &len);
 		if (len != 1) {
 			PyErr_BadArgument();
 			return -1;