Make Adaptor AdaptorClosure and nodes part of public interface

[YarikTH]

Description

Currently all the stuff required to write your own adapters is hidden under private ureact::detail namespace. It means that it is unstable and can be silently changed every day. Some adapters can be written in terms of existing adapters, but it is quite limiting.

So, to allow user to freely extend the library, we need to expose at least all required building blocks. They are no needed by common inexperienced user, so adding them directly in ureact namespace is not convenient. Need to add additional namespace. Let's say ureact::core.