[WIP] allow all functions to be passed to Clump.source(...)

@fwbrasil this doesn't compile. I was just trying to allow and subclass of iterable to be used in Clump.source. I have the generics working however I don't know what to do when we actually invoke the fetch function, since we have to convert from a `Set[T]` to an `In`. I think you can do some magic with `CanBuildFrom` stuff but it's a bit over my head and now I am too tired to keep looking at it today :)

I still think it would be nice to have the flexibility here, WDYT?