ListT proposal for `transformers`

[Gabriel Gonzalez]

> It seems there are 140 packages on hackage using ListT (which surprised
> me, given its flaws), so I don't think we can quickly remove it from
> transformers or re-implement it under the same name

Don't worry.  I've decided to keep it under `pipes` anyway.  It seems 
like a more natural fit for it to be in `pipes` because it is otherwise 
very hard to build or consume `ListT`-done-right computations.  So 
consider this proposal to be retracted.