Allow Data instances for external data types
Currently, the frontend only allows custom data instances in the prelude. It should also allow data instances for external data types in every module. Moreover, the frontend currently does not check whether there is another Data instance in the Prelude for normal data types. That way, it is possible to have two Data instances at once for one data type. This issue would be fixed by allowing custom Data instances only for external data types.
Edited by Finn Teegen