[Ur] The future of [class] declarations

[Karn Kallio]

> 
> So, one alternative would be this: Remove [class] declarations but keep
> [class] signature items.  This means type classes would only be a
> concept in the module system.  Automatic resolution would only happen
> for a type class hidden behind opaque signature ascription of a module,
> though instances could then be declared elsewhere in the current way.
> This is actually pretty close to what is de facto implemented right now,
> so perhaps the [class] declaration is just a distraction.
> 

Perhaps a better alternative is to make instances get picked up in the
current module (preserving the class declaration as it is).