use default template parameter for Tuple2

Could change declaration of Tuple2 to this:

template<class T1, class T2 = T1>
class Tuple2
{
...
}:

This would make it simpler, less verbose to use something like Tuple2<label> instead of Tuple2<label, label> for places where we would normally use Pair<label> but don't want the automatic binary output associated with Pair (inherited from FixedList).

Alternative? Derive Pair from Tuple2 ... not sure about that one.