stt-0.2.1: A monad transformer version of the ST monad

A monad transformer version of the ST monad Warning! This monad transformer should not be used with monads that can contain multiple answers, like the list monad. The reason is that the state token will be duplicated across the different answers and this causes Bad Things to happen (such as loss of referential transparency). Safe monads include the monads State, Reader, Writer, Maybe and combinations of their corresponding monad transformers.