Add an instance MonadFix Initializer

[ocharles]

It looks like Initializer is just a combination of reader and state monads, so it should be possible to get a MonadFix instance out of this. I would like to use this as we have our Initializer split over two modules and MonadFix gives me a nice way to "share" some parameters between them. I'll see if I can provide a pull request soon.