Eta-reduce the type synonyms in Plain.hs to allow use in transformer stacks

Because you cannot partially apply type synonyms, the currently-exported Reactive type synonym can't be used as the base of a monad transformer stack:

    > type M s = StateT s Reactive

    <interactive>:6:1:
        Type synonym `Reactive' should have 1 argument, but has been given none
        In the type declaration for `M'

Eta-reducing the definitions removes this limitation.  I don't think it changes any existing usage.