Refactor Jafar monad

[tsani]

The Jafar monad never actually makes use of IO, so there's no reason to stack our transformers over the IO monad; we could stack them over the Identity monad. In fact, the refactoring should make a JafarT monad transformer that we can choose to stack over anything we like, and we ought to make a MonadJafar class whose methods are the particular operations enabled by including JafarT in a monad transformer stack. These methods should probably just be the haskell implementations of the high-level functions available in the Jafar EDSL.