Consider using injector constructor kwargs as values scoped to the instance.

[groner]

We've seen how Injector.value() can be problematic, and injector already has an instance scoped values dict. But we lack an API for populating it.

Injector.sub() uses kwargs to create instance scoped values. I think it would be useful for the injector constructor to do the same.

This would enable us to use a static request injector class in our dispatch layer at MN, which would be much simpler.