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/bltinmodule.c b/Python/bltinmodule.c
index 27a3d8c..ae7ceec 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -1606,7 +1606,7 @@
 				Py_DECREF(stdin_encoding);
 				return NULL;
 			}
-			prompt = PyUnicode_AsString(po);
+			prompt = _PyUnicode_AsString(po);
 			if (prompt == NULL) {
 				Py_DECREF(stdin_encoding);
 				Py_DECREF(po);
@@ -1639,7 +1639,7 @@
 			else {
 				result = PyUnicode_Decode
 					(s, len-1,
-					 PyUnicode_AsString(stdin_encoding),
+					 _PyUnicode_AsString(stdin_encoding),
 					 NULL);
 			}
 		}