Accept functions as parameter / return

[alexbarcelo]

There is no way to pass a functional thingy as a parameter to a dataClay method (or as a return of it).

This includes lambda, functions or methods.

Some of those are not serializable (like lambda) and adding the feature may be challenging (like closures related to functions) and not a whole lot usefeul. However, supporting references to methods of registered classes seems a little more manageable and may be suited for certain use cases.

We may add this feature for all or a subset of functional thingies.