Re: #4159: move Monad and MonadFix instances for Either from mtl to base

Sorry for being inarticulate in haste, a couple of typo corrections: The version that Ross puts forward is the version that can make sense inside

of Base. It fits with the module it is moving into, it doesn't require Base adopting whole-hog the Control.Monad.Error.Class machinery, and it drops [an instance] for a [prelude type] into the [module] where it belongs.

In that vein it makes sense that the basic error type used by

transformer-centric [code] should ALSO be a type alias wrapped around ErrorT, rather than an abuse of a prelude type.

-Edward Kmett