# Description
- What does this PR do?
1. Upgrades `trie-db`'s version to the latest release. This release
includes, among others, an implementation of `DoubleEndedIterator` for
the `TrieDB` struct, allowing to iterate both backwards and forwards
within the leaves of a trie.
2. Upgrades `trie-bench` to `0.39.0` for compatibility.
3. Upgrades `criterion` to `0.5.1` for compatibility.
- Why are these changes needed?
Besides keeping up with the upgrade of `trie-db`, this specifically adds
the functionality of iterating back on the leafs of a trie, with
`sp-trie`. In a project we're currently working on, this comes very
handy to verify a Merkle proof that is the response to a challenge. The
challenge is a random hash that (most likely) will not be an existing
leaf in the trie. So the challenged user, has to provide a Merkle proof
of the previous and next existing leafs in the trie, that surround the
random challenged hash.

Without having DoubleEnded iterators, we're forced to iterate until we
find the first existing leaf, like so:
```rust
        // ************* VERIFIER (RUNTIME) *************
        // Verify proof. This generates a partial trie based on the proof and
        // checks that the root hash matches the `expected_root`.
        let (memdb, root) = proof.to_memory_db(Some(&root)).unwrap();
        let trie = TrieDBBuilder::<LayoutV1<RefHasher>>::new(&memdb, &root).build();

        // Print all leaf node keys and values.
        println!("\nPrinting leaf nodes of partial tree...");
        for key in trie.key_iter().unwrap() {
            if key.is_ok() {
                println!("Leaf node key: {:?}", key.clone().unwrap());

                let val = trie.get(&key.unwrap());

                if val.is_ok() {
                    println!("Leaf node value: {:?}", val.unwrap());
                } else {
                    println!("Leaf node value: None");
                }
            }
        }

        println!("RECONSTRUCTED TRIE {:#?}", trie);

        // Create an iterator over the leaf nodes.
        let mut iter = trie.iter().unwrap();

        // First element with a value should be the previous existing leaf to the challenged hash.
        let mut prev_key = None;
        for element in &mut iter {
            if element.is_ok() {
                let (key, _) = element.unwrap();
                prev_key = Some(key);
                break;
            }
        }
        assert!(prev_key.is_some());

        // Since hashes are `Vec<u8>` ordered in big-endian, we can compare them directly.
        assert!(prev_key.unwrap() <= challenge_hash.to_vec());

        // The next element should exist (meaning there is no other existing leaf between the
        // previous and next leaf) and it should be greater than the challenged hash.
        let next_key = iter.next().unwrap().unwrap().0;
        assert!(next_key >= challenge_hash.to_vec());
```

With DoubleEnded iterators, we can avoid that, like this:
```rust
        // ************* VERIFIER (RUNTIME) *************
        // Verify proof. This generates a partial trie based on the proof and
        // checks that the root hash matches the `expected_root`.
        let (memdb, root) = proof.to_memory_db(Some(&root)).unwrap();
        let trie = TrieDBBuilder::<LayoutV1<RefHasher>>::new(&memdb, &root).build();

        // Print all leaf node keys and values.
        println!("\nPrinting leaf nodes of partial tree...");
        for key in trie.key_iter().unwrap() {
            if key.is_ok() {
                println!("Leaf node key: {:?}", key.clone().unwrap());

                let val = trie.get(&key.unwrap());

                if val.is_ok() {
                    println!("Leaf node value: {:?}", val.unwrap());
                } else {
                    println!("Leaf node value: None");
                }
            }
        }

        // println!("RECONSTRUCTED TRIE {:#?}", trie);
        println!("\nChallenged key: {:?}", challenge_hash);

        // Create an iterator over the leaf nodes.
        let mut double_ended_iter = trie.into_double_ended_iter().unwrap();

        // First element with a value should be the previous existing leaf to the challenged hash.
        double_ended_iter.seek(&challenge_hash.to_vec()).unwrap();
        let next_key = double_ended_iter.next_back().unwrap().unwrap().0;
        let prev_key = double_ended_iter.next_back().unwrap().unwrap().0;

        // Since hashes are `Vec<u8>` ordered in big-endian, we can compare them directly.
        println!("Prev key: {:?}", prev_key);
        assert!(prev_key <= challenge_hash.to_vec());

        println!("Next key: {:?}", next_key);
        assert!(next_key >= challenge_hash.to_vec());
```
- How were these changes implemented and what do they affect?
All that is needed for this functionality to be exposed is changing the
version number of `trie-db` in all the `Cargo.toml`s applicable, and
re-exporting some additional structs from `trie-db` in `sp-trie`.

---------

Co-authored-by: Bastian Köcher <[email protected]>