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

[Edward Z. Yang]

Excerpts from Isaac Dupree's message of Fri Jul 02 12:25:33 -0400 2010:
> It's not quite as simple as that -- as soon as we start making Either an 
> instance of kind (* -> *) classes, such as Functor and Monad, there 
> inevitably becomes some kind of semantic difference between Left and Right.

Right, so what we end up with is short-circuiting computation with different
results.

Cheers,
Edward