investigate again D performance

[glathoud]

Reason: not so interesting for a simple use case, where ldc2 -O is already doing a great job to optimize map/filter etc.: https://github.com/glathoud/transfun/tree/master/misc/d_performance_feasability

but there could be more interesting, slightly more complex use cases where transfun is interesting. At least interesting to try and know the answer.