Compatibility with mtl-2.3

[sjakobi]

With --allow-newer=mtl, I'm seeing a build failure:

Building library for zio-0.1.0.2..
[1 of 2] Compiling Paths_zio        ( dist/build/autogen/Paths_zio.hs, dist/build/Paths_zio.o, dist/build/Paths_zio.dyn_o )
[2 of 2] Compiling ZIO.Trans        ( src/ZIO/Trans.hs, dist/build/ZIO/Trans.o, dist/build/ZIO/Trans.dyn_o )

src/ZIO/Trans.hs:30:35: error:
    Not in scope: type constructor or class ‘ExceptT’
   |
30 | newtype EIO e a = EIO { _unEIO :: ExceptT e UIO a }
   |                                   ^^^^^^^

src/ZIO/Trans.hs:31:57: error:
    Not in scope: type constructor or class ‘MonadFix’
    Perhaps you meant ‘MonadFail’ (imported from Prelude)
   |
31 |   deriving ( Functor, Applicative, Monad, MonadError e, MonadFix, Unexceptional )
   |                                                         ^^^^^^^^

src/ZIO/Trans.hs:34:57: error:
    Not in scope: type constructor or class ‘MonadFix’
    Perhaps you meant ‘MonadFail’ (imported from Prelude)
   |
34 |   deriving ( Functor, Applicative, Monad, MonadError e, MonadFix, MonadReader r, Unexceptional )
   |                                                         ^^^^^^^^

Note that the removal of ExceptT is due to haskell/mtl#117 and a future version of mtl is expected to restore this re-export.