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

[Henning Thielemann]

On Tue, 29 Jun 2010, Edward Z. Yang wrote:

> 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.

+1