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

[Ross Paterson]

On Thu, Jul 01, 2010 at 11:35:24AM +0100, Simon Marlow wrote:
> I don't know all the ins and outs of the discussion here, but before
> proposing to reverse this decision someone should do a careful
> assessment of the prior rationale and results of subsequent
> experience.

The general question of fail is a side issue -- the question here is
just whether the Monad instance for Either should define fail, and if
so to what.