differential-dataflow-v0.18.0

Other

• Updates for timely 0.25 (#647)
• Replace IndexMap with BTreeMap (#652)
• Update columnar; introduce mimalloc
• Extract VecCollection from Collection (#651)
• Introduce traits for collection containers (#650)
• Correct some clippy nit (#649)
• Batcher implementation that has no opinions about chains, and columnar. (#626)