Fix node child overwrites and string export

[chrisjenkinson]

This fixes two problems in the node model:

• addNode() throws DuplicateNodeException when a child with the same name already exists, instead of silently replacing it. The new replaceNode() swaps out an existing child in place and throws NodeNotFoundException if there isn't one. getNode() now throws that exception too, and it's still a RuntimeException. NodeTraverser uses replaceNode() when a visitor returns a node with the same name. A series of same-type children goes in an array attribute, which the traverser already visits.
• __toString() exports the whole tree. Before, every child node came out as {}. NodeInterface now extends JsonSerializable, and invalid UTF-8 in attributes is replaced with U+FFFD instead of causing a TypeError.

Breaking changes: NodeInterface gains replaceNode() and jsonSerialize(), and adding a second child with the same name now throws.