Remove Snapshot

[gsserge]

Due to its Copy-on-Write nature, creating a snapshot of the tree is just a matter of cloning it, e.g. tree.clone().

Some of the snapshot tests are moved to the art.rs module with small modifications.

[arriqaaq]

Can we add this test back just to verify behaviour of SI

    #[test]
    fn snapshot_isolation() {
        let mut tree: Tree<VariableSizeKey, i32> = Tree::<VariableSizeKey, i32>::new();
        let key_1 = VariableSizeKey::from_str("key_1").unwrap();
        let key_2 = VariableSizeKey::from_str("key_2").unwrap();
        let key_3_snap1 = VariableSizeKey::from_str("key_3_snap1").unwrap();
        let key_3_snap2 = VariableSizeKey::from_str("key_3_snap2").unwrap();

        assert!(tree.insert(&key_1, 1, 0, 0).is_ok());

        // Keys inserted before snapshot creation should be visible
        let mut snap1 = tree.clone();
        assert_eq!(snap1.get(&key_1, 0).unwrap(), (1, 1, 0));

        let mut snap2 = tree.clone();
        assert_eq!(snap2.get(&key_1, 0).unwrap(), (1, 1, 0));

        // Keys inserted after snapshot creation should not be visible to other snapshots
        assert!(tree.insert(&key_2, 1, 0, 0).is_ok());
        assert!(snap1.get(&key_2, 0).is_none());
        assert!(snap2.get(&key_2, 0).is_none());

        // Keys inserted after snapshot creation should be visible to the snapshot that inserted them
        snap1.insert(&key_3_snap1, 2, 0, 0).unwrap();
        assert_eq!(snap1.get(&key_3_snap1, 0).unwrap(), (2, 2, 0));

        snap2.insert(&key_3_snap2, 3, 0, 0).unwrap();
        assert_eq!(snap2.get(&key_3_snap2, 0).unwrap(), (3, 2, 0));

        // Keys inserted after snapshot creation should not be visible to other snapshots
        assert!(snap1.get(&key_3_snap2, 0).is_none());
        assert!(snap2.get(&key_3_snap1, 0).is_none());
    }

[gsserge]

@arriqaaq added the SI test