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/Modules/readline.c b/Modules/readline.c
index 98da33b..8371942 100644
--- a/Modules/readline.c
+++ b/Modules/readline.c
@@ -724,7 +724,7 @@
 			result = NULL;
 		}
 		else {
-			char *s = PyUnicode_AsString(r);
+			char *s = _PyUnicode_AsString(r);
 			if (s == NULL)
 				goto error;
 			result = strdup(s);