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

[Edward Kmett]

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
>
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