Introduce special collection instance functions

[beikov]

When #28 is done, we can introduce some special collection instance functions like the following:

• any - Returns any element
• min - Returns the minimum element, optional ordering argument?
• max - Returns the maximum element, optional ordering argument?
• size - Returns the size of the collection
• sum - Returns the sum of the collection elements, optional Function argument to extract value to sum?
• avg - Returns the average of the collection elements, optional Function argument to extract value to average?

Note though, that it would be nice if we could add the functions to specific collection types only i.e. use a receiver type of Collection[UserType] so that the function is only available for types that really support this.
This kind of also depends on #30