chore: collapse node4 to child and get rid of node 1

[arriqaaq]

Description

As per the ART paper, when a Node4 has a single child should have its child collapsed into it. Currently this is not implemented, and rather a Node4 is collapsed into a Node1 which doesn't really help in compression. This PR implements delete as per the paper.

Some benchmark results

# main

seq_delete/vart         time:   [1.3538 µs 1.3743 µs 1.3913 µs]
                        thrpt:  [718.76 Kelem/s 727.64 Kelem/s 738.67 Kelem/s]

rand_delete/vart        time:   [2.4015 µs 2.4515 µs 2.5083 µs]
                        thrpt:  [398.68 Kelem/s 407.92 Kelem/s 416.40 Kelem/s]

# chore/collapse-node4-to-child

seq_delete/vart         time:   [1.1587 µs 1.1767 µs 1.1914 µs]
                        thrpt:  [839.36 Kelem/s 849.85 Kelem/s 863.07 Kelem/s]


rand_delete/vart        time:   [1.8899 µs 1.9467 µs 2.0162 µs]
                        thrpt:  [495.99 Kelem/s 513.70 Kelem/s 529.12 Kelem/s]