Added `copy` and `isEmpty` methods and `+` operator to Lists.

Added `copy` and `isEmpty` methods and `+` operator to Lists. Added tests. Made them pass, except for adding lists to ranges, e.g. `[1,2,3] + (4..6)`. Implementing `count` and `isEmpty` methods on Ranges would make that happen, as ranges were working before I started checking for empty lists. Alternatively, List.fromRange class method or a Range.toList method could be useful, but I'm not sure how to do that yet.