NormalizedNode.toString() changed

Controller is using a rather unfortunate toString() conversion during JSON export.
The output has changed for ImmutableLeafNode, as we now have two distinct implementations, whereas previously we used ImmutableLeafNode for normal data and a subclass for byte[] data. This leads to toString() saying 'Regular[]' instead of 'ImmutableLeafNode[]'.

Add a way to override the reported class and provide overrides for LeafNode and LeafSetEntryNode implementations.