Remove unused vars in Tree tests.
Tree: Add traverseNode() and findNode() methods.

…along with matching traverse() and find() wrapper methods on Tree.Node.

traverseNode() passes the specified node and each of its descendants to a
callback function in depth-first order. Traversal can be stopped by returning
`Tree.STOP_TRAVERSAL` from the callback.

findNode() passes the specified node and each of its descendants to a
callback function and returns the first node for which the callback returns a
truthy value.
Merge branch 'dev-3.x' into tree-traverse

Conflicts:
	src/tree/HISTORY.md
Build tree.