Support for complex mapping adaptation

[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. Some scenarios can be supported by derived attributed inserted through aspects. In a lot of other cases, an OCL-like expression can express the relationship between features of two different DSLs.

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