Add hash proc for cstrings (#5386)

web/news/e031_version_0_16_2.rst

@@ -18,7 +18,11 @@ Changes affecting backwards compatibility
 - The IO routines now raise ``EOFError`` for the "end of file" condition.
   ``EOFError`` is a subtype of ``IOError`` and so it's easier to distinguish
   between "error during read" and "error due to EOF".
-
+- A hash procedure has been added for ``cstring`` type in ``hashes`` module.
+  Previously, hash of a ``cstring`` would be calculated as a hash of the
+  pointer. Now the hash is calculated from the contents of the string, assuming
+  ``cstring`` is a null-terminated string. Equal ``string``s and ``cstring``s
+  produce equal hash value.
 
 Library Additions
 -----------------