Added mutable operator[] for integral types (you can only grow the sequence if you specify the *next* element)

Major switch from Value -> Node. The library compiles with the new API, but tests are still oldies, and don't compile

Updated the node/value classes with the new iterators, they compile until we try to instantiate anything

Made the 'data' member optional in node_ref - it's only created on-demand, so we don't waste extra memory every time we do Value tmp = v;

Switched value = otherValue to assign the actual nodes after setting the reference (so that tmp = foo['bar']; tmp = other; is the same as foo['bar'] = other;)