SessionManager is weird

[mikeplus64]

Currently SessionManager is an existential type that wraps around something that has an instance of ISessionManager. But no instances of ISessionManager are actually exposed, despite the docs linking to a "CookieSession", so it's the existential is a slightly more inefficient way of writing

data SessionManager r = SessionManager
  { load :: r -> Snap r
  , commit :: r -> Snap ()
  , ...
  }

But that isn't very satisfying either.