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

[Ross Paterson]

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.

Discussion deadline: 20th July 2010