Well core.async is a Clojure library so it uses features available in Clojure. I don't see how it would affect the fact that it is a library.

I've also linked to js-csp, a JS library obviously not implemented using macros.

I can also find other examples of implementation as libraries, but I have no experience with them:

- Scala: https://github.com/rssh/scala-gopher

- F#: https://github.com/Hopac/Hopac

- C++: http://www.cs.kent.ac.uk/projects/ofa/c++csp/