move Monad and MonadFix instances for Either from mtl (proposal #4159)
The Monad and MonadFix instances for Either (formerly in the mtl package) are moved to Control.Monad.Instances and Control.Monad.Fix respectively. The Monad instance is still an orphan, to retain Haskell 98 compatibility, but the MonadFix instance is together with its class. The Error constraint is removed from both instances, and the default definition of fail is used.
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