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/setobject.c b/Objects/setobject.c
index 771f47e..1054166 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -2390,7 +2390,7 @@
 	/* Exercise direct iteration */
 	i = 0, count = 0;
 	while (_PySet_NextEntry((PyObject *)dup, &i, &x, &hash)) {
-		s = PyUnicode_AsString(x);
+		s = _PyUnicode_AsString(x);
 		assert(s && (s[0] == 'a' || s[0] == 'b' || s[0] == 'c'));
 		count++;
 	}