Give MonadTrans a QuantifiedConstraints superclass

[Henning Thielemann]

On Thu, 3 Jun 2021, Fumiaki Kinoshita wrote:

> +1 to the original proposal of using QuantifiedConstraints.
> 
> There is no need to stick to the standard from 23 years ago, and having 
> two different classes is only likely to bring confusion and extra work 
> for library maintainers.

But why do we have the separation between 'transformers' and 'mtl' then?