Sort the tree from node and place results in sorted.

Property definitions

trees $ BinTreeMap :: sort_down
	# Sort the tree from `node` and place results in `sorted`.
	protected fun sort_down(node: N, sorted: Array[E]) do
		if node.left != null then sort_down(node.left.as(not null), sorted)
		sorted.add(node.value)
		if node.right != null then sort_down(node.right.as(not null), sorted)
	end
lib/trees/bintree.nit:320,2--325,4