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

[Edward Z. Yang]

Excerpts from Ross Paterson's message of Tue Jun 29 05:26:49 -0400 2010:
> The proposal is to move the Monad and MonadFix instances for Either
> (currently in the mtl package) to Control.Monad.Instances and
> Control.Monad.Fix respectively (both in the base package).  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.

+1, and long overdue, I think.

Cheers,
Edward