Use MTL: Accum, State, SelectT, ContT

* [ ] Use AccumT for `(w -> m (a, w))` parser:
* https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-Accum.html
* [ ] Use StateT for `(s -> m (a, s)) ` parser:
* https://hackage.haskell.org/package/mtl-2.3.1/docs/Control-Monad-State-Strict.html#t:State
* [ ] Use SelectT for `((a -> m r) -> m a)` selecting:
* https://hackage.haskell.org/package/transformers-0.5.6.2/docs/Control-Monad-Trans-Select.html#t:SelectT
* [ ] Use ContT for `(a -> m r) -> m r` continuation:
* https://hackage.haskell.org/package/transformers-0.5.6.2/docs/Control-Monad-Trans-Cont.html#t:ContT
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* [x] Change name `Unit` to `Automaton`