AttributeResolver and Resolver API

[Tom Zeller]

> So I suspect it might not be too tortured to do something similar with the
> AttributeResolver, but only if it made additional sense, which I'm not
> sure it does. In particular the Resolver API really just has the resolve()
> and resolveSingle() methods, while I'm guessing the AttributeResolver is
> defined to populate a subcontext to return the attributes. So I'm still
> not sure it makes any sense. Just wanted to note it.

Agreed. Also noting that the ResolverPlugin might be a natural fit to
extend Resolver.