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/unicodeobject.c b/Objects/unicodeobject.c
index 5925f80..5bf0fa2 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -1452,7 +1452,7 @@
 }
 
 char*
-PyUnicode_AsStringAndSize(PyObject *unicode, Py_ssize_t *psize)
+_PyUnicode_AsStringAndSize(PyObject *unicode, Py_ssize_t *psize)
 {
     PyObject *bytes;
     if (!PyUnicode_Check(unicode)) {
@@ -1468,9 +1468,9 @@
 }
 
 char*
-PyUnicode_AsString(PyObject *unicode)
+_PyUnicode_AsString(PyObject *unicode)
 {
-    return PyUnicode_AsStringAndSize(unicode, NULL);
+    return _PyUnicode_AsStringAndSize(unicode, NULL);
 }
 
 Py_UNICODE *PyUnicode_AsUnicode(PyObject *unicode)