Visit all children or self

Property definitions

markdown2 $ MdNode :: visit_all
	# Visit all children or `self`
	fun visit_all(v: MdVisitor) do
		var node = first_child
		while node != null do
			var next = node.next
			v.visit(node)
			node = next
		end
	end
lib/markdown2/markdown_ast.nit:127,2--135,4