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;)

Switched memory to using shared nodes, and node_data to keep only naked node pointers, not shared nodes (to break the cycle, and we don't need weak pointers because their memory is guaranteed to exist, via 'memory')

Added (another) layer - now 'node_ref' is between node and node_data, and it decrees whether nodes are identical