Provides a generic, cookie-based middleware for sessions that is
        parameterised over the session store, the cookie name, and the
        cookie parameters (such as path, expiry, etc).  Passes a pair of
        functions (lookup key, set key) for the current session through the
        'Vault' in the 'Request'.
        .
        Also provides a simple example session store based on threadsafe
        'IORef's and 'Data.Map'.
        .
        See example/Main.hs in git for example usage.