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

[Ross Paterson]

On Wed, Jul 21, 2010 at 04:46:09PM -0700, Iavor Diatchki wrote:
>  Picking appropriate names for things makes a surprisingly big
> difference when reading large amounts of code, especially when you are
> not the program's author.

That's a useful principle, but few follow it all the way to its logical
conclusion.  It's also useful to have a few standard types, to provide
a common interface between user functions and lots of utility functions.
That's why we have lists and pairs, and Maybe and Either.