Add iterator over ProgTree elements

[Copilot]

Implements an iterator over ProgTree that traverses elements following the progressive tree structure defined in EIP-7916.

Implementation

• ProgTreeIter<'a, T>: Iterator that maintains a single reference to the current progressive node and wraps iter::Iter instances for each binary subtree
• Traversal order: Visits right children in sequence—root's right child, then right child of each left node down the spine
• iter(&self, length: usize) -> ProgTreeIter<'_, T>: Public API on ProgTree

Example

let mut tree = ProgTree::<u64>::empty();
for i in 1..=65 {
    tree = tree.push(i, i - 1).unwrap();
}

// Iterate all elements in progressive tree order
for value in tree.iter(65) {
    println!("{}", value);  // Prints 1, 2, 3, ..., 65
}

The iterator handles both packed (u64, u8) and non-packed (Hash256) types. The implementation uses a single reference that follows the left spine sequentially, making it simple and memory-efficient.

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