implement iterator for blocktreenode

this makes more sense as a useful iterator because block ids are
copy. This can be used to implement mutable/immutable reference
iteration for NotePage.

src/blocktree.rs

@@ -38,6 +38,42 @@ impl BlockTreeNode {
     }
 }
 
+struct BlockTreeIterator<'a> {
+    node_stack: Vec<&'a BlockTreeNode>
+}
+
+impl<'a> BlockTreeIterator<'a> {
+    fn new(root_node: &'a BlockTreeNode) -> Self {
+        Self {
+            node_stack: vec![root_node]
+        }
+    }
+}
+
+impl<'a> Iterator for BlockTreeIterator<'a> {
+    type Item = i128;
+    fn next(&mut self) -> Option<Self::Item> {
+
+        // stack is empty, we are done otherwise continue with current_node
+        let Some(current_node) = self.node_stack.pop() else {
+            return None;
+        };
+
+        // if the current node has a next sibling, stack it.
+        if let Some(next_sibling_node) = &current_node.next_sibling_node {
+            self.node_stack.push(next_sibling_node);
+        }
+
+        // if the current node has a first child, stack it.
+        if let Some(first_child_node) = &current_node.first_child_node {
+            self.node_stack.push(first_child_node);
+        }
+
+        // yield a clone of the block pointed to by current_node
+        Some(current_node.block_id)
+    }
+}
+
 #[derive(Debug)]
 pub struct NotePage {
     title: String,