#274 Evaluate explicite types of method parameters and return types using...

#274 Evaluate explicite types of method parameters and return types using Types#asMemberOf
I'm wondering in which cases the variants of getReturnType, getParameters, getThrownTypes with the ExecutableElement are to be used instead of the ExecutableType.

My guess is, we only need the EcecutableElement variant when we render the implementations of the mapping methods. But whenever we do checks on the types, we would use the ExecutableType variant. Is that assumption correct? Are we doing it the right way in all places now?

Perhaps we should reflect the difference of the variants in a more clearly separated naming of the methods? Such as ```getEffectiveReturnType(ExecutableType ..)``` vs. ```getDeclaredReturnType(ExecutableElement ..)```. Those methods could then also be moved to our ```Method``` interface (without the parameters then, just as simple getters), with some explanations in the javadoc.

WDYT?