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.