Proposal to solve the `EitherT` problem.

[Ross Paterson]

On Tue, Aug 13, 2013 at 10:30:23AM -0400, Edward Kmett wrote:
> An argument against just randomly bikeshedding the name it is there
> are a lot of packages out there currently transitively depending on
> the existing either package, due to the popularity of Tekmo's errors
> package and the fact that it has been picked up by snap. So half of
> the web-apps in the ecosystem depend on this type transitively.

Fortunately it seems that EitherT is only used by the following packages:

        citation-resolve coroutine-object CSPM-Frontend errors
        happstack-heist hoodle-core hoodle-parser katt pdf-toolbox-core
        pianola restricted-workers terminfo-hs

Moreover adding a new module and type means people can switch over an
extended timescale.  Thus I think internal consistency within transformers
outweighs compatibility with the existing EitherT in this case.