Support for renaming

[barais]

(split from #7)

Name-based structural typing works well for evolution. However, mapping independently developed DSLs rarely works out-of-the-box.

Melange should support adaptation specification, at least through renaming, ideally with advanced mapping expressions.

First step is to include renaming expression

In such a case, it is the role of the adapters to effectively realize the adaptation. The renaming expression must be compiled in the adapter's code.