Proposal: merge either into transformers

[Chris Wong]

A couple of these are in transformers already.

> Combinators which I am used to using at work include:
>
>    firstEitherT :: Functor m => (x -> y) -> EitherT x m a -> EitherT y m a

https://hackage.haskell.org/package/transformers-0.5.2.0/docs/Control-Monad-Trans-Except.html#v:withExceptT

>    secondEitherT :: Functor m => (a -> b) -> EitherT e m a -> EitherT e m b

This is just fmap

-- 
Chris Wong (https://lambda.xyz)

"I had not the vaguest idea what this meant and when I could not
remember the words, my tutor threw the book at my head, which did not
stimulate my intellect in any way." -- Bertrand Russell