Expose API to allow to build custom backends

Stream: general

[Serhii Khoma (srghma)]

It would be cool to build JavaScript backend and Dart backend (in idris it is easier) or clojure backend (idris transpires to clojure) or even rust backend.

But the problem is:

1. Environment that comes to emitter is very c++ oriented
2. C++ is a hard to understand language
3. There are no examples how to generate code for other environments

[Kim Morrison]

This is not on the roadmap.