Tweak ReadStateT MonadTransControl instance for GHC 8.2

[enolan]

GHC 8.2.1 is more strict in the way type variables can be used in associated
types. The error is:

/home/enolan/code/signalvine/monad-classes.hs/Control/Monad/Classes/ReadState.hs:42:8: error:
    • Type indexes must match class instance head
      Expected: StT (ReadStateT x) <tv>
        Actual: StT (ReadStateT s) a
      where the `<tv>' arguments are type variables,
      distinct from each other and from the instance variables
    • In the type instance declaration for ‘StT’
      In the instance declaration for ‘MonadTransControl (ReadStateT x)’
   |
42 |   type StT (ReadStateT s) a = StT IdentityT a
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This commit fixes it.