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/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index e65a9a4..dbd5314 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -95,7 +95,7 @@
 	const char *str;
 
 	assert(PyUnicode_Check(errors));
-	str = PyUnicode_AsString(errors);
+	str = _PyUnicode_AsString(errors);
 	if (str == NULL)
 		return NULL;
 	cb = PyCodec_LookupError(str);
@@ -148,7 +148,7 @@
 		return -1;
 	}
 
-	str = PyUnicode_AsString(value);
+	str = _PyUnicode_AsString(value);
 	if (str == NULL)
 		return -1;