Make map work with set (and others)

[roetlich]

This is my attempt to solve #11. I added a call to toList to the map function that is being called by the for comprehension. That allows for to be used with a lot of things, such as lists, sets, tuples and even strings.

This does solve the issue that for needs to work with sets, but it might be too much. Maybe iterating over strings isn't intended?